Trans-base tunnel reader for sequencing

ABSTRACT

A device for reading the sequence of a polymer, consisting of a first electrode and a second electrode, each electrode being functionalized with a reader molecule strongly bonded to the electrodes, but forming weak bonds with a molecule to be sequenced. In particular, the reader molecule is designed to form bonds with at least two points on the target molecule such that the target molecule is trapped between a first reader molecule on one electrode and a second reader molecule on the second electrode, with the overall size of the molecular complex being small enough to permit significant electric current to flow.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a §371 National Stage Application ofPCT/US2009/059693 filed Oct. 6, 2009 which claims priority to U.S.provisional application 61/103,019, filed on Oct. 6, 2008 and PCTapplication, PCT/US2009/037563, filed on Mar. 18, 2009, andPCT/US2009/37570, filed on Mar. 18, 2009.

GOVERNMENT RIGHTS

This invention was made with government support under NHGRI Grant No.1R21 HG004378-01 and 5R21 HG004378-02 awarded by the National Instituteof Health. The government has certain rights in the invention.

FIELD OF THE INVENTION

The present invention is directed to systems, devices and methods foridentifying biopolymers, such as strands of DNA, as they pass through aconstriction such as a carbon nanotube nanopore. More particularly, theinvention is directed to such systems, devices and methods in which anewly translocated portion of the biopolymer forms a transientelectrical circuit between the nanotube nanopore and a second electrode,which may also be a nanotube. Further, the invention is directed to suchsystems, devices and methods in which the constriction is provided witha functionalized unit which, together with a newly translocated portionof the biopolymer, forms a temporary electrical circuit that can be usedto characterize that portion of the biopolymer.

BACKGROUND OF THE INVENTION

Several approaches have been employed or proposed to perform DNAsequencing. Among the various approaches, the nanopore approach has thegreat advantage of only allowing one base to pass a particular point ata time (if the orifice is small enough). It can also be highlyprecessive (moving from one base to the next without “stuttering”) ifthe driving force is high enough.

Nanotubes have been considered as one way to implement a nanopore. Forinstance, the use of carbon nanotubes as nanopores through which DNA maybe translocated electrophoretically has been described in T. Ito, L.Sun, R. M. Crooks, Chemical Communications, 1482 (2003) (Ref 1). Silicananotubes have also been used as nanopores, as reported in R. Fan etal., Nano Letters 5, 1633 (September, 2005). A new approach for readingthe sequence of a DNA molecule passing between electrodes on a nanoporeusing hydrogen bond-mediated tunneling signals has been proposed in J.He, L. Lin, P. Zhang, S. Lindsay, Identification of DNA base-pairing viatunnel current decay. Nano Letters 7 (12), 3854-3858, 2007. U.S. Pat.No. 6,821,730 discloses the use of carbon nanotube probes to sequenceDNA. U.S. Pat. No. 6,627,067 discloses a method of translocatingmolecules through nanopores for sequencing purposes.

The aforementioned articles and patents are incorporated by reference tothe extent necessary to understand the present invention.

SUMMARY

In one aspect, the present invention is directed to a readout device andscheme for DNA sequencing through a constriction, such as a nanopore.The device is adapted so that a DNA strand can translocate through thenanopore of a first nanotube (“translocating nanotube”). As the DNAstrand emerges from the translocating nanotube, a portion of the strandcompletes a temporary electrical circuit between the translocatingnanotube and a second electrode, which may comprise a nanotube (“contactnanotube”). The electrical circuit utilizes the electron tunnelingcurrent mediated by specific hydrogen-bonding molecular recognitionevents between portions of the DNA strand and functionalized entitiesattached to the two nanotubes.

The scheme utilizes the electron tunneling current mediated by specifichydrogen-bonding molecular recognition events.

In another aspect, the present invention is directed to the design andconstruction of a manufacturable instrument, constructed so as to allowfor parallel operation of many constrictions for performing sequencing,such as of ssDNA or dsDNA.

The system employs at least one device having at least two sensingelectrodes spaced apart by a gap and positions on either side of aconstriction, such as a nanopore. The nanopore electrode gapconstruction may be achieved by electrochemical assembly to produce gapsthat are reformable in-situ. Alignment of a nanogap sensing electrodepair with a constriction is achieved by means of novel ‘though-pore’plating process. Thereafter, active gap control may be used todynamically-control the gap. Since the natural DNA bases frequently formmismatched base pairs, custom recognition elements (referred to hereinas “affinity elements”) are used for molecular recognition. Eachconstriction is functionalized with at least one such custom affinityelement. Electrophoresis, magnetic bead technology and the signal fromthe pore itself can be used to effect translocation through theconstriction and characterization of the molecule. The system is thusconfigured to acquire data related to the locations of specific bases ina single strand of DNA.

In the device, a pair of spaced apart sensing electrodes border on theconstriction. The first sensing electrode is connected to a firstaffinity element (e.g., a phosphate grabber when the target molecule isssDNA) while the second sensing electrode is connected to a secondaffinity element. Each affinity element may be connected to itscorresponding electrode via one or more intermediary moieties, such as amolecular linker, which itself typically is connected to the electrodevia an electrode anchoring group, such as a thiol. The first and secondaffinity elements are configured to temporarily form hydrogen bonds withfirst and second portions of the molecule as the latter passes throughthe constriction. During translocation, the electrodes, affinityelements and first and second portions of the target molecule completean electrical circuit and allow a measurable electrical current to passbetween the first and second electrodes. The time-varying nature of thiselectrical current, and the specific affinity elements employed, allowone to characterize the first and second portions of the targetmolecule.

The present invention's approach to nanopore electrode construction isdirected to mimicking the scanning tunneling microscopy that has provedeffective and successful in experiments with hydrogen-bond-basedelectronic recognition. Three elements of this are: 1) self-alignedmetal-gap-metal junctions capable of being reformed in-situ; 2) activecontrol of the tunnel gap; and 3) manufacturability. The metal used inthese junctions can be gold. Trials with gold electrodes have indicatedthat the “blinking” of contacts made to soft metals is not a significantproblem.

The present invention further provides an embodiment where the firstrecognition element comprises a base reader as discussed above and thesecond recognition element comprises a base pair reader instead ofphosphate grabber. This provides a new method for formingchemically-specific chemical contacts to single stranded DNA, by formingself-assembled hydrogen bonded contacts across each base. PreferredG-C/C-G base pair readers include, but are not limited to cinnolinederivatives described herein. Preferred A-T/T-A base pair readersinclude, but are not limited to 1,8-napthyridine derivatives and1,10-phenanthroline derivatives described herein.

The present invention provides a device for characterizing a biopolymer.The device has a carbon nanotube with an interior channel through whichthe biopolymer is translocated. Preferably the translocation is drivenby electrophoresis. The carbon nanotube comprises a plurality of gapscut into the carbon nanotube. Each of the plurality of gaps isfunctionalized to provide a characteristic signal at each of theplurality of gaps. Preferably the biopolymer is a nucleic acid such asDNA or RNA and the carbon nanotube comprises four gaps cut into thecarbon nanotube, wherein each gap is functionalized with a differentbase reader.

The present invention also provides a device for determining thesequence of a nucleic acid comprising: a) a solid surface to support acarbon nanotube having an interior channel through which the nucleicacid can travel; and b) an insulating film layer disposed on top of thecarbon nanotube.

The insulating film layer is preferably PMMA. The film layer comprisesan origination, a first, a second, a third, a fourth and a terminationwell capable of containing an electrolyte. The origination well isproximal to the first, second, third, fourth and termination wells, andthe termination well is distal to the first, second, third, fourth andorigination wells.

The carbon nanotube comprises a first, second, third and fourth gap cutinto the carbon nanotube and each of the gaps contact a separate well.For example, the first gap contacts the first well, the second gapcontacts the second well, the third gap contacts the third well and thefourth gap contacts the fourth well.

Each of the gaps of the carbon nanotube has a first gap end and a secondgap end; and each of the first gap ends is functionalized with a firstrecognition element, and each of the second gap ends is functionalizedwith a second recognition element.

The carbon nanotube has an origination end contacting the originationwell and a termination end contacting the termination well, and anorigination electrode contacts the origination well and a terminationelectrode contacts the termination well. The origination and terminationelectrodes do not contact the carbon nanotube.

The device further comprises a first electrode contacting the carbonnanotube between the origination well and the first well; a secondelectrode contacting the carbon nanotube between the first and secondwell; a third electrode contacting the carbon nanotube between thesecond and third well; a fourth electrode contacting the carbon nanotubebetween the third and fourth well; and a fifth electrode contacting thecarbon nanotube located between the fourth and termination well;

In certain embodiments, the solid surface comprises an oxidized siliconwafer and the carbon nanotube is grown directly on the silicon wafer.

In certain embodiments the nucleic acid is DNA or RNA and the firstrecognition element comprises a phosphate grabber such as guanidiniumand the second recognition element comprises a nucleobase reader, suchas a nucleobase reader that specifically recognizes its Watson-Crickbase pair complement.

The present invention also provides a method of making devices of thepresent invention. A carbon nanotube is grown on a silicon wafer. Aplurality of electrodes are deposited on the silicon wafer and contactthe carbon nanotube. A plurality of electrodes are deposited on thesilicon wafer that do not contact the carbon nanotube. A plurality ofgaps having a first and second gap end are cut into the carbon nanotube.Each gap is cut so that it is located between two of the plurality ofelectrodes that contact the carbon nanotube. First and secondrecognition elements are conjugated to each of the plurality of firstgap ends and second gap ends, respectively.

The present invention also provides a method of detecting a tunnelcurrent signal that switches between two levels, which is characteristicof an interaction with a single base, and wherein said signal is used toidentify the target base in the tunnel gap. The present invention alsoprovides a method of identifying a target base in a tunnel gap bydetecting a tunnel-current signal that switches between two levels,wherein the signal switch is characteristic of an interaction with asingle base.

The present invention also provides a molecular recognition chipconfigured to identify at least one portion of a target molecule, thechip comprising:

a substrate; a first nanotube passing through at least a portion of thesubstrate, the first nanotube provided with a nanopore and configuredand dimensioned to permit the translocation of a target moleculetherethrough; a second nanotube supported by the substrate and disposedin a fixed relationship with respect to said first nanotube so as toform a first gap with the first nanotube; and an electrical circuitconfigured to detect an electrical current between the first and secondnanotubes, upon passage of the target molecule past said gap, wherein:the first nanotube comprises a first recognition element connected to anend thereat and the second nanotube comprises a second recognitionelement connected to a first end thereof, and wherein the first andsecond recognition elements comprise a universal base reader.

The invention also provides a device for obtaining an electrical signalcharacteristic of a DNA base pair comprising a first universal basereader attached to a first electrode and a second universal base readerattached to a second electrode, wherein the universal base readers arecapable of recognizing different DNA bases and are also capable offorming an additional set of hydrogen bonds with a complex of a DNA baseand the universal base reader, and wherein the universal base readerscomprise a conjugate of a planar π system with a functional group,amine, amide, nitro, carboxylic acid, and halogen, connected to amolecular linker.

The invention also provides a device for obtaining an electrical signalcharacteristic of a DNA base pair comprising a universal base readerattached to a first electrode and a universal base reader attached to asecond electrode, wherein the universal base reader is capable ofrecognizing different DNA bases (including methylated DNA bases) and isalso capable of forming an additional set of hydrogen bonds with acomplex of a DNA base and the universal base reader. In certainembodiments, the universal base reader comprises1H-imidazole-2-carboxamide containing a molecular tether, such as2-aminoethyl or mercaptomethyl, at its 4 position.

The present invention also provides a method of sequencing DNA as theDNA molecule translocates through a constriction, the method comprising:a) providing an apparatus comprising a DNA sequencing device inaccordance with claim 2, the device being located in the apparatus suchthat a first chamber is located on the first side of the device and asecond chamber is located on the second side of the device; b)introducing the DNA molecule into the second chamber; c)electrophorescing the DNA molecule so that it translocates through theconstriction; and d) detecting an electrical current passing through thefirst electrode, a first universal base reader, a nucleo base of the DNAmolecule, a second universal base reader, and the second electrode.

The present invention also provides a composition comprising a universalbase reader wherein the reader is selected from the group consisting of5-(2-aminoethyl)-1H-imidazole-2-carboxamide.

The present invention also provides a method of synthesizing theuniversal base readers.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention and to show how thesame may be carried out in practice, reference will now be made to theaccompanying drawings.

FIG. 1 shows the overall structure of a device in accordance with oneembodiment of the present invention, with the molecular recognition chipshown in vertical cross-section.

FIG. 1A is an enlarged view of a portion of FIG. 1 showing the tunnelgap between the translocation nanotube and the contact nanotube.

FIGS. 2A-2M illustrate different stages in the fabrication of the chipof FIG. 1, in accordance with one embodiment of the present invention.

FIG. 3 illustrates one embodiment of a process for functionalizing anend of a nanotube with guanidinium.

FIG. 4 illustrates one embodiment of a process for aligning a contactnanotube relative to a translocating nanotube.

FIGS. 5 and 5A: Sequencing by recognition through affinity elements,showing one of four types of reader (this one is for C). A guanidiniumion tethered to one electrode via a flexible linker, hydrogen bonds(yellow H-bonds) onto the nearest passing phosphate on an ssDNAtranslocating a nanopore. If a flexibly tethered base on a secondelectrode finds it is a Watson-Crick complement on the other side of theDNA (red H-bonds) a large current passes between the two electrodes,signaling a base recognition event. The components require an electrodegap of about 3 nm and an electrode height of no more than 0.6 nm or 0.7nm. The H-bonding also serves to align the DNA in the device, while theflexible linkers provide alignment tolerance. Translocation iscontrolled via electrophoresis and magnetic beads (with net force F) anarrangement compatible with a parallel assembly of many reading heads.

FIG. 6A-6B: FIG. 6A shows a high-resolution imaging of a prior artnanogap. FIG. 6B shows a prior art nanogap sculpted by e-beam ablation(from Fischbein and Drndić, 2006 and 2007).

FIG. 7A-7D: Testbed nanogap made by lithography and FIB. FIG. 7A shows aschematic layout, including a covering layer of SiO₂. FIG. 7C shows across section of gap.

FIG. 7B shows a SEM image of a real device with another view into thenanogap shown in FIG. 7D.

FIG. 8A-8B: (FIG. 8A) i-v plots for tunnel devices (as-made) similar tothat shown in FIG. 7. (FIG. 8B) Current vs. time after closing the gapselectrochemically and then stripping them open. Quantum-conductancesteps (indicated by arrows) are clearly observed as Au is removed.

FIG. 9: Scheme for through-pore plating (showing a nanopore made by TEMshrinkage as an inset, lower right). The key feature is through-poretransport of Au+ ions, localizing deposition to parts of the sensingelectrodes (SE1, SE2) in close proximity to the pore. Metal depositionand stripping is controlled by the built in counter electrode (CE) usingthe built-in reference (RE) with the sensing electrodes serving asworking electrodes (operated at a small potential difference, V_(t)).VEC sets the potential of the working electrodes. Measurements of porecurrent (I_(P)) and tunnel-current between the two working electrodes(I_(t)) is used as control parameters for final pore size and tunnel-gapsize. The two data sets together can be used to center the electrodes inthe pore.

FIG. 10A-10C: Models for finite element analysis. 10A-10D model of theelectrodeposition setup. 10B—A close-up including the double layer(EDL). 10C—Full 3D model of the electrodeposition setup including EDLstructure.

FIG. 11. Structure of a PNA trimer composed of modified uracil anduniversal bases.

FIG. 12. Base pairing of the cytosine reader (R_(C)) with natural DNAbases.

FIG. 13. Proposed structures of modified guanines for improvingspecificity of the C reader.

FIG. 14. Base pairing of the guanine reader (R_(G)) with natural DNAbases.

FIG. 15. Basepairing of the G-clamp with guanine.

FIG. 16. Base pairing of DAP with DNA bases and proposed analogues ofDAP as candidates for the T reader.

FIG. 17. A universal DNA base reader (R_(U)): hydrogen bonding schematicfor 4-(mercaptomethyl)-1H-imidazole-2-carboxamide.

FIG. 18: Magnetic bead apparatus. The CCD can track a bead being pulledinto the nanopore to within 10 nm. Inset (upper right) is the prototypelaboratory apparatus.

FIG. 19A-19C: (a) Forces on a molecule with bead stretching andelectrophoretic translocation. (b) Bead arrangement for ‘flossing’experiment. (c) Magnetic force added to electrophoretic force.

FIG. 20 shows an embodiment of a device in which the constriction infound in a microfluidic channel formed on a surface of the device.

FIGS. 21A and 21B show an embodiment of a device in which theconstriction is a pore through a substrate and the electrodes compriselayers along the thickness of the pore.

FIGS. 22A and 22B show an electrode comprising chemically depositedlayers of conducting metal.

FIG. 23 shows an exemplary electrical arrangement of a device inaccordance with one embodiment of the present invention.

FIG. 24 shows one embodiment of the invention where a G-C or C-G basepair reader forms a triple hydrogen bond with the G base present on theDNA strand and the C base reader attached to one electrode.

FIG. 25 shows one embodiment of the invention where a A-T or T-A basepair reader forms a triple hydrogen bond with the T base present on theDNA strand and the A base reader attached to one electrode.

FIG. 26 shows an exemplary device with the DNA translocated through ananopore and a base reader attached to one electrode and a base pairreader attached to a second electrode.

FIG. 27: A serial recognition sequencer

FIG. 28: Oxidized silicon wafer bearing a carbon nanotube.

FIG. 29: Carbon nanotube patterned with electrodes

FIG. 30: Device after masking, formation of wells and oxygen plasmaetch.

FIG. 31: Wells cut into a second PMMA film.

FIG. 32: Selective functionalization of one side of the pairs of readingelectrodes with the phosphate grabber (“PG”).

FIG. 33: Functionalization of the remaining side of the readers with theadenine reader (AR), thymine reader (TR), cytosine reader (CR) andguanine reader (GR).

FIG. 34; Construction of a CNT nanopore device (A). (B) is SEM image(PMMA pegs prevent collapse of PDMS) Enhanced contrast region showsSWCNT in orange (prior to plasma etch). (C) Complete device with PDMScover.

FIG. 35: Special mode of transport for DNA in a “tight” nanotube: (A)current through 5 nm MWCNT prior to DNA addition and (B) after additionof 0.1 nM 60 nt Oligo. (C) 2 nm SWCNT signal prior to DNA addition andafter addition (D) of same DNA as above, and shows current over a 10minute interval (vertical lines are 2 minute markers). This tube gaveabout a spike (red arrows) per minute. The unstable background ischaracteristic of DNA addition and not seen for salt alone. Tubes lessthan 2 nm diameter give no translocation. Bias=0.2V3 electrolyte is 2MKCl.

FIG. 36: Many molecules translocate per current spike. (A) Typical qPCRsignal for controls and two different translocation times. Product isverified by gels and also direct sequencing. (B) Number of moleculestranslocated vs. number of spikes. Uncertainties are owing variablefilter performance and cut-off criteria for counting spikes. The slope(100 molecules per spike) is clearly much larger than one.

FIG. 37: Wetted CNTs conduct. (A) Device with pair of Pd electrodescontacting CNT under PMMA barrier. (B) Current through CNT as a functionof back-gate voltage for dry (red, black) and wet (green blue) tube.

FIG. 38: Trans-base-pair readers in action. The readers (red) areattached to one electrode with a base (blue) attached to the other. AWatson-Crick complement on the target DNA strand results in complexstabilized by 6 hydrogen bonds for G, C and T targets and 5 for an Atarget. The H-bond structures shown are the lowest energy structuresfound in quantum-chemical simulations. The arrangements shown here arefor a T-target (A) and a G-target (B). An A-target is read by replacingthe 2AA on the second electrode with T in (A) and a G-target is read byreplacing the C on the second electrode with G in (B). Operation of theG reader is illustrated by a calculation of the conductance of thejunction with, and without the G target-base in (C) (the gold slabsshown constitute part of the structure projected into semi-infiniteelectrodes). The relative lifetimes of the bound and unbound complexesare expected to differ significantly too.

FIG. 39: Showing two sites (red, green) for amide linkages on an 18,0CNT (of the required 2 nm diameter). A total of 36 sites are availablein this model.

FIG. 40 provides Scheme 1 to synthesize a base-pair reader.

FIG. 41 provides Scheme 2 to synthesize a base-pair reader.

FIG. 42 Telegraph noise measurements on trans-base-pair readers. (A)Without target and (B) simulated current-time signal. (C) After captureof a thymine nucleotide with (D) simulated signal.

FIG. 43 shows a device of the present invention showing feedbackcircuitry for controlling translocation of ssDNA through the CNT.

FIG. 44 provides a three well device for comparing translocation out ofor across a small gap.

FIG. 45 provides a device with electrical contacts used to probe theeffects of reading bias across the gap.

FIG. 46 Selective attachment of DNA base reader (R) by electrochemicaloxidation at one of the electrode ends of a hydroquinone-functionalizedCNT gap.

FIG. 47: Heterogeneous junction without EBL; (A) A thin layer ofAluminum is patterned. (B) Exposed CNT is etched by O₂ plasma withnanometer over-etch and Al is slightly oxidized. (C) Structure is coatedwith Pt to form a Pt electrode facing the CNT across a gap. (D) Metalelectrode is patterned overlapping with Al. (E) Oxide etch, lift off,leaving CNT-metal nanometer gap, the Al will be oxidized again to forminsulating Alumina oxide layer. (F) SEM showing 4.5 nm junction. (G)Corresponding tunnel characteristics confirming gap size.

FIG. 48: (a) An intact junction in which the tunnel gap is spanned by aguanine attached to the probe, hydrogen bonded to a deoxycytidineattached to the substrate. (b) Fluctuations that break themetal-molecule contact will reduce the conductance, as will fluctuationsthat break the hydrogen bonds (c). (d) An example of the telegraph noisesignal produced as bonds break and reform.

FIG. 49: Recordings of tunnel current vs. time (left column) togetherwith the corresponding distributions of current for (a) a controljunction with thiophenol on the probe and thymidine on the surface, (b)adenine on the probe and thymidine on the surface, (c) 2-aminoadenine onthe probe and thymidine on the surface and (d) guanine on the probe anddeoxycytidine on the surface. Solid lines are Gaussian fits to the upperand lower switching level distributions. These fits are used, togetherwith the bias, to determine the molecular switching conductance in agiven run. High-current switching data are given in FIG. 53.

FIG. 50: Plot of molecular switching conductance vs. baselineconductance for the three base-nucleotide combinations (a-c). (d)illustrates mechanisms for the various regions. For (1) the tunnel gapis larger than the equilibrium length of the molecular pair, leading toa region of rapid increase in conductance (shaded gray in a-c) as thestrain required to span the gap decreases. When the gap is equal to orsmaller than the equilibrium length of the molecular pairs, they mayspan the gap in either the equilibrium configuration (3—shaded green ina-c, θ=0) or tilted configurations (2—unshaded data in a-c). The appliedbiases were 0.05V (squares), 0.1V (circles) and 0.2V (diamonds).

FIG. 51: Distribution of “on” times for G-deoxycytidine (left). Thedistribution is plotted as a function of G(bl) in the 2D color plot onthe right (red=high counts). Parameters obtained from Gaussian fits(τ_(F), τ_(S) and h_(F)/h_(S)) are listed in Table 2. Data for the otherbase-nucleoside pairs are given FIG. 54.

FIG. 52: Current-voltage curves for 2AA-thymidine (diamonds),A-thymidine (circles) and G-deoxycytidine (squares). Each data point isthe mean obtained from data with conductances >0.5 nS. The error barscorrespond to ±1 sd.

FIG. 53 provides high current switching data.

FIG. 54 provides data for additional nucleoside pairs.

FIG. 55 provides a synthesis scheme for a universal base reader.

FIG. 56 provides a schematic diagram of DNA base recognition with theuse of two universal base readers (designated as Ur). Pairs ofelectrodes, each functionalized with a universal base reader, will trapeach of the bases in complexes that form a tunneling path across theelectrode.

FIG. 57: provides examples of compounds that can function as a universalreader.

FIG. 58 a provides the structure of an exemplary modified adenosine foruse in an organic solvent (the OH groups in the deoxyribose ring werefunctionalized with t-butyldimethylsilyl) (TBDMS). FIG. 58 b providesthe structure of one universal base reader, thiobenzoic acid.

FIG. 59 provides the results of a study detailed in Example 6. FIG. 59(a) shows the tunnel current in pure solvent and FIG. 59( b) shows tunnelcurrent after the addition of 500 nM Guanidine. The inset shows a blowup of a typical peak—the residence times are typically 0.5 to 1 ms.

FIG. 60 is a histogram of current spike amplitudes as nucleosides passthrough a tunnel gap. Dependence of mean current and the width of thedistribution on the tunnel gap for (A) cytidine and (B) Thymidine. AtG_(bl)=4 pS (black) there are very few reads. As G_(bl) is increasedfrom 12 pS (dark grey) to 20 pS (light grey) the reads move to highervalues and the distribution broadens. The light grey background is whathappens with no functionalization of probe and surface. The dark greydistributions are for the functionalization shown—in aprotic solventCOOH is one part H bond acceptor (═O) and one part donor (—OH). So thisone very simple reagent forms H bonds with both bases to complete thecircuit across the (now much larger) tunnel gap. There is no signal atall without nucleosides present in solution, and the dark greydistributions show that one can clearly distinguish C from T in singlemolecule reads with amazing fidelity on one read in one gap.

FIG. 61( a) provides spike height distributions for all four nucleosidesand 5-methyl cytidine. (b) Blow up of the data for adenosine (lightgrey) with the distribution for adenine superimposed (dark grey) showingthe reproducibility and demonstrating that interactions are dominated bythe base.

FIG. 62 shows tunnel current distribution without functionalizedelectrodes for thymidine (dark grey) and cytidine (light grey).

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to nanopore based DNA sequencing. Suchsystem utilized the concept of sequence by recognition: use ofhydrogen-bond mediated chemical recognition to transduce an electricalsignal for a recognized base. See PCT/US08/59602, filed Apr. 7, 2008,which is herein incorporated by reference. In addition to sequence byrecognition, the present inventors have developed additional inventionswhich are described and claimed herein, namely, a trans-base tannerreader for sequencing, the use of single-walled carbon nanotubes(SWCNTs) as nanopores, and the integration of the electrode system intothe SWCNT itself, which simplifies the manufacture of the reader.Described herein are various devices of the invention, methods of makingthose devices, recognition elements (such as phosphate grabbers, basereaders and base-pair readers) useful in the devices and variousexperiments performed to show the feasibility of using such devices.

Device Utilizing a Translocation Nanotube and a Contact Nanotube forTunneling Current Formation.

One embodiment of the present invention provides a device 100 shown inFIG. 1. The device 100 includes a molecular recognition chip 102connected to an electrical measurement circuit 104. In one embodiment,the electrical measurement circuit 104 is configured to measure atunneling current as discussed further below.

In one embodiment, the chip 102 comprises a substrate 112, a firstinsulating layer 114 formed over the substrate 112, a first metalcontact layer 116 formed over the first insulating layer 114, an secondinsulating layer 118 formed over the first metal contact layer 116, anda second metal contact layer 120 formed over the second insulating layer118. In one embodiment, the second metal contact layer 120 may bering-shaped. In one embodiment, the substrate 112 is formed fromsilicon, the first insulating layer 114 is an oxide layer (e.g., silicondioxide) and the second insulating layer 118 is a silicon nitride (SiN)layer. The first and second metal contact layers 116, 120 are formedfrom a material capable of forming an ohmic contact with a nanotube. Inone embodiment, palladium is used for the first and second metal contactlayers 116, 120. Palladium is known to form ohmic contacts with carbonnanotubes, which are used in conjunction with the chip 102 as describedfurther below.

The chip 102 has a upper surface 130 and a lower surface 132. Thevarious layers of the chip 102 made be etched to result in the shapeshown in FIG. 1 using techniques known to those skilled in the art.

A first nanotube 240 (the “translocation nanotube”) is formed through athickness of the chip 102. The translocation nanotube 240 has a lowerend 242 and an upper end 244 connected by a through channel 146.Proximate its lower end 242, outer walls of the translocation nanotube240 make an ohmic contact with the first metal contact layer 116.Proximate its upper end 244, outer walls of the translocation nanotubeare surrounded by the insulating material from the second insulatinglayer 118. The through channel 146 of the translocation nanotube 240provides the chip 102 with a nanopore 146 through which molecules, suchas DNA, can translocate. In one embodiment, the translocation nanotube240 is a carbon nanotube (CNT).

In one embodiment, a second nanotube 250 (the “contact nanotube”) isformed on the second insulating layer 118 at a location where the secondinsulating layer forms a portion of the chip's upper surface 130. Thecontact nanotube 250 has a first end 252 that is separated by a gap 190(a “tunnel gap”) from the upper opening 244 of the translocationnanotube 240. The contact nanotube 250 also has a second end 254 whichforms an ohmic contact with a portion of the second metal contact layer120. As stated above, the second metal contact layer 120 may have aring-shape, and so may partially or entirely encircle the translocationnanotube's upper end 244. In one embodiment, the contact nanotube 250 isalso a carbon nanotube (CNT). In some embodiments, the translocationnanotube 240 and the contact nanotube 250 are substantially identical instructure.

A first lead 160 of the electrical measurement circuit 104 contacts thefirst metal contact layer 116 (which forms an ohmic contact with thelower end of the translocation nanotube 240). A second lead 162 of theelectrical measurement circuit forms an ohmic contact with the secondmetal contact layer 120 (which forms an ohmic contact with the secondend of the contact nanotube). Thus, when the tunnel gap 190 between thetranslocation nanotube's upper end 244 and the contact nanotube's firstend 252 is occupied by an electrically conductive moiety, such as aportion of a DNA strand, a measurable current flows through theelectrical measurement circuit 104.

In one embodiment, the device 100 is configured to read one of the basesas a DNA strand 170 passes from a lower side 180 of the chip 102 to anupper side 190 of the chip 102, through the translocation nanotube 240.One nanotube is used for translocating DNA while the second nanotube isused to contact the DNA and generate an electrical signal. The contactnanotube is functionalized for recognition of a particular nucleotide.It is therefore understood that multiple such chips 102 may be requiredso that all nucleotides may be read as identical DNA strandssimultaneously translocate through chips havingdifferently-functionalized contact nanotubes.

As a DNA strand 170 passes through the translocating nanotube 240, itwill form a first set of bridging hydrogen bonds between its phosphatesand a phosphate grabber molecule attached to the end of thetranslocating nanotube 240. It will also form a second set of bridginghydrogen bonds between its bases and a recognition element tethered tothe contact nanotube 250, thereby completing an electrical circuit, butonly if a nucleotide on the DNA strand is in some sense complementary tothe recognition element (e.g., a recognition ‘base’) tethered to thecontact nanotube 250. In one embodiment, the phosphate grabber moleculecomprises a guanidinium moiety and the recognition element is a modifiedbase complementary to the base on the DNA strand.

It is understood that to make use of the device 100, the device 100 mustbe mounted in an assembly configured to cause a DNA strand to pass thelower side 180 of the chip 102 to the upper side 190 of the chip 102. Inone embodiment, such an assembly may be provided with electricalcircuitry that causes the DNA strand to electrophoresceorese through thetranslocation nanotube 240. Magnetic beads or the like may affixed to aleading end of the DNA strand so as to properly orient the DNA strand inpreparation for translocation.

FIGS. 2A-2M illustrate one embodiment for fabricating the device 100seen in FIG. 1

As seen in FIG. 2A, one first provides suitable silicon substrate 112having alignment marks for future processes. Then, a first insulatinglayer 114 is formed over the silicon substrate. In one embodiment, thefirst insulating layer 1114 is formed by growing an oxide layer 114 ontop of the silicon layer 112. In one embodiment, the oxide layer 114 hasa thickness of about 10 nm.

As seen in FIG. 2B, the next step is to provide one or more verticallyoriented carbon nanotubes 240 (which will later be used as translocationnanotubes) on the first insulating layer 114. The carbon nanotubes 240are provided at low density on the insulating layer 114. In oneembodiment, the vertically oriented carbon nanotubes are to grown. Daihas reported arrays of 2 nm Fe or Co metals (By EBL) as seeds forsingle-walled nanotube (SWNT) growth. However, under normal conditions,micron long single-walled carbon nanotubes (SWNTs) lie on the surface.An alumina template may be needed to get the CNTs to grow vertically onthe substrate. Another choice is to grow short multi-walled nanotubes(MWNT). Furthermore, silica nanotubes may also be a candidate for thispurpose.

As seen in FIG. 2C, the next step is to form a first metal contact layer116 over the first insulating layer 114. In one embodiment, palladium(Pd) or a palladium-gold (Pd/Au) alloy is deposited on top of the oxidelayer so as to form a metallic ohmic contact to the translocationnanotube 240. Palladium has been shown to be a suitable contact materialfor metallic SWNTs. In one embodiment, a 5 to 10 nm layer of Pd or Pd/Aualloy is deposited on the first insulating layer 114.

As seen in FIG. 2D, the next step is to form a second insulating layer116 over the first metal contact layer 114. In one embodiment, thesecond insulating layer comprises a layer of silicon nitride (SiN). Inone embodiment the silicon nitride is deposited on top of the firstmetal contact layer 114 to a thickness of about 50 to 100 nm layer. Thesilicon nitride acts as both an electrical insulator (relative to thefirst metal contact layer) and also acts as a fluid barrier between topand bottom surfaces of the final device 100.

As seen in FIG. 2E, the next step is to perform an “etch back” removethe extra portion of the translocation nanotube 240 protruding above thesurface of the second insulating layer 116. In one embodiment, this maybe accomplished with a hydrogen plasma etch or anodic electrochemicaletching. As a result of this step the upper surface of the secondinsulating layer 116 may be smoothed.

As seen in FIG. 2F, the next step is to functionalize the translocationnanotube 240 with a carboxylic acid group 222. In one embodiment, thismay be accomplished by etching for 2 hours in a 6M H₂SO₄, 2M HNO₃solution.

As seen in FIG. 2G, the underside of the silicon substrate 112 is etchedto form a window 294. In one embodiment, this may be accomplished by KOHetching under a desired portion of the structure. In one embodiment thewindow 294 size is chosen based on the density of nanotubes and thefinal probability of success, given the yield at each stage of thefabrication process. Specifically, if the probability of forming asuccessful reading junction is pj, and the number of nanotubes per unitarea is N/A, then the desired area of the final chip, Am is given byAm=A/(pj×N). This will result in, on average, one properly connected,functioning junction per chip. The remainder of the first insulativelayer 114 proximate the window, and metal particles used for seeds forgrowing the translocation nanotube 240 are removed by reactive ionetching. This can expose the lower end 242 of the translocation nanotube240.

As seen in FIG. 2H, the translocation nanotube 240 functionalized withone or more carboxylic acid groups 222 is reacted with a guanidiniumgroup 210 tethered to a primary amine. When reacted with the carboxylategroup on the CNT by means of a zero-length cross-linker such as EDC 212(1-ethyl-3-[3-dimethylaminopropyl]carbodiimide hydrochloride), theresult is an amide bond, thereby resulting in a carbon atom 244anchoring a guanidinium group 242, as seen in FIG. 3.

Separately, a separate batch of micron-long metallic CNTs (which willeventually serve as contact nanotubes) are carboxylated and then reactedwith one of the bases (or other recognition elements). These recognitionelements are also connected to a primary amine to facilitate attachmentto the carboxylated end of nanotubes using EDC or other couplingreagents.

As seen in FIG. 2I, a sacrificial nucleotide 270 (see FIG. 4) isconnected to the assembly on the silicon nitride surface 118 containingthe guanidinium-functionalized nanotube 240. This sacrificial nucleotideis complementary to the recognition element that was tethered to thecontact nanotube discussed immediately above. This is done so that upperends 244 of the translocation nanotube 240 can become bound to the firstend 252 of a carbon nanotube, via phosphate to guanidinium hydrogenbonds.

As seen in FIG. 2J, contact nanotubes 250, which are tethered torecognition elements, are flowed over the silicon nitride surface 118surface containing the guanidinium-bound nucleotides tethered to thetranslocation nanotubes 240. As a result, some of the translocationnanotubes 240 will become bound to contact nanotubes 250 bearingrecognition elements via hydrogen bonding with the recognition element260. The overall arrangement of one type of hydrogen-bonded nucleotideis shown in FIG. 4. Other devices for recognizing other bases will beassembled with the appropriate nucleotides and recognition elements.This can be done on a wafer having multiple chips and appropriatemasking to control the assembly.

As seen in FIG. 2K, a protective passivation layer 282 is placed overthe structure to cover the active area, in preparation for thesubsequent steps. In one embodiment, the passivation layer covers atleast the translocation nanotube 240 and the contact nanotube 250.

As seen in FIG. 2L, a second metal contact layer 120 is formed such thatit contacts the second end 254 of the carbon nanotube 250. In oneembodiment, the second metal contact layer 120 is formed from the samematerials as the first metal contact layer (for instance, formed from Pdor a Pd/Au alloy). In one embodiment, the second metal contact layer isformed in the shape of a ring. The ring-shape facilitates an ohmiccontact to the contact nanotube, and also facilitates the formation ofcontacts to electrical leads 160, 162 of an electrical measurementcircuit configured to measure a tunneling current between thetranslocation nanotube 240 and the contact nanotube 250.

Finally, as seen in FIG. 2M, the passivation layer 282 is removed, as isthe sacrificial nucleotide 270 used as an alignment template. In oneembodiment, these two items are removed with acid. This leaves thedesired gap 190 for bases on a DNA strand to be trapped via thephosphates by the translocation nanotube 240 and via the complementaryrecognition element on the contact nanotube 250.

FIG. 3 shows the process for functionalizing a nanotube with guanidinium210 to form the translocation nanotube 242. One or more guanidiniummolecules 210 are attached to an available carboxyl group 222 (—COOH)formed on the end of a carbon nanotube 220. Techniques for attachingguanidinium to a carboxyl group are known to those of ordinary skill inthe art. This results in a carbon nanotube 240 in which guanidiniummoieties 242 are tethered to the carbon atoms of what formerly wereavailable carboxyl groups 222 on the original carbon nanotube 220.

Similarly, one of four bases (A, C, T or G) derivatives can be tetheredto an available carboxyl group on a carbon nanotube to form afunctionalized recognition element of the contact nanotube. In thismanner, the contact nanotube serves as a functionalized recognitionnanotube which is adapted to recognize a nucleotide complementary to thebase tethered thereto.

During sequencing operations, as the DNA strand 170 translocates throughthe nanopore of the translocation nanotube 240, it emerges from thelatter's upper end 244. As this happens, a phosphate from the backboneof the emerging DNA strand 170 forms a first set of temporary hydrogenbonds with a guanidinium moiety 242. At roughly the same time, anucleobase of the emerging DNA strand 170 forms a second set oftemporary hydrogen bonds with the functionalized recognition elementbelonging to the contact nanotube. When both sets of bonds form, adetectable tunneling current is created and measured by the electricalmeasurement circuit 104. Analysis of the time-varying magnitude of thedetected tunneling current can provide information about thenucleotide's identity. The principle behind this paradigm foridentifying nucleotides are discussed in J He, Jin, Lin, Lisha, Zhang,and Lindsay. Identification of DNA base-pairing via tunnel currentdecay. Nano Letters 7 (12), 3854-3858, 2007, whose contents areincorporated by reference.

FIG. 4 illustrates the process for aligning the first end 252 of thecontact nanotube 250 on the upper surface 130 of the chip 102, relativeto the upper end 244 of the translocation nanotube 240. As seen in FIG.4, guanidinium moieties 242 are tethered to the upper end 244 of thetranslocation nanotube 240 while a base 260 (in this instance, cytosine)is tethered to the first end 252 of the contact nanotube 250. Thecytosine base 260 serves as the functionalized recognition element 260for the contact nanotube 250.

The alignment technique relies on self-alignment of the contact nanotube250 relative to the translocation nanotube 240, with the assistance of asacrificial nucleotide 270. In the shown embodiment, the sacrificialnucleotide 270 is deoxyguanosine monophosphate. The sacrificialnucleotide is introduced so that its phosphate group 272 forms a firstset (in this instance, a pair) of hydrogen bonds with one of theguanidinium moieties 242. The sacrificial nucleotide's forms a secondset of hydrogen bonds with the functionalized recognition element 260belonging to a contact nanotube 250. The position of the contactnanotube 240 is adjusted to align and form this second set of hydrogenbonds, thereby creating an optimal tunnel gap 190 between the contactnanotube 250 and the translocation nanotube 240. It is understood thatthe precise distance of the tunnel may vary slightly from chip to chip,depending on the exact configuration and morphology of the guanidiniummoieties 242 and the base 260. Once alignment has been completed, thesacrificial nucleotide 270 may be removed and the chip 102 prepared foruse in DNA sequencing.

It is understood that the association of the guanidinium moieties 242with the translocation nanotube 240 and the functionalized recognitionelement 260 with the contact nanotube 250 is a matter of design choice.One may equally configure the chip 102 such that the translocationnanotube 240 is provided with the functionalized recognition element 260while the contact nanotube 250 is provided with the guanidinium moieties242. In other embodiments, the translocation nanotube may befunctionalized with a recognition element, such as a base (A, G, C or T)or a base reader and the contact nanotube may be functionalized with abase-pair reader as disclosed herein below. Alternatively thetranslocation nanotube may be functionalized with a base pair reader andthe contact nanotube may be functionalized with the base or base readerrecognition element.

Furthermore, in another embodiment, instead of using self-alignment, anactive positioning device is used to position the contact nanotube 250.In this embodiment, the contact nanotube 250 would be attached to ananopositioning device, such as the scanning head of a scanningtunneling microscope or atomic force microscope, and moved into positionby using electronic tunneling between the two nanotubes as an indicatorof their proximity.

The present invention uses chips provided with both a translocationnanotube and a functionalized contact nanotube, in conjunction with andhydrogen bonding for molecular recognition. Since the natural DNA basesfrequently form mismatched base pairs, custom recognition elements areused for molecular recognition and so each contact nanotube isfunctionalized with at least one such custom recognition element.

Thus, in one embodiment, a device may be used to sequence a strand ofDNA by the following set of principal steps:

Step 1. Provide a wafer having a plurality of chips 102 of the sort seenin FIG. 1. The wafer should have at least four types of chips, each typehaving a contact nanotube functionalized to recognize one of the fourbases, should be provided. Instead of single wafer, one may insteadprovide an apparatus having the four types of chips separately mountedtherein.

Step 2. Place the wafer in an apparatus configured to accommodate DNAsequencing, with copies of the DNA strand to be sequenced present on thelower side of each such chip. Optionally, the DNA may be modified so asto allow entry into the translocation nanotube from one direction only.In one embodiment, this may be done by tethering the DNA to a bead.

Step 3. Electrophorese the DNA strands through the nanopore of thetranslocation nanotubes of each chip, in synchrony. If extra pullingforce is needed, functionalize the end that passes through pore (afterhaving been modified with e.g., biotin) and attach magnetic bead.

Step 4. Pull DNA through by electrophoresis and/or magnetic bead.

Step 5. Record current pulses (I_(t)) as a function of time as the DNAstrands emerge from the nanopore of the translocation nanotube and formtemporary hydrogen bonds with the guanidinium and the differentrecognition elements attached to the contact nanotube, thereby creatinga detectable tunneling current.

Step 6. Align data from a plurality of reads for each type of basereader.

Step 7. Align data from all 4 reads.

Although the present invention has been described to a certain degree ofparticularity, it should be understood that various alterations andmodifications could be made without departing from the scope of theinvention as hereinafter claimed.

Device Utilizing a Trans Base Tunnel Reader

FIG. 5 shows one embodiment of a device for sequencing single-strandedDNA (ssDNA) by hydrogen-bonding recognition, in accordance with thepresent invention. In its simplest form, each recognition-molecule(referred to as a ‘base-reader’) reads a specific DNA base, the fullsequence being assembled by juxtaposing data from four differentreaders. As the ssDNA passes the electrodes via a constriction (e.g., ananopore), a guanidinium ion grabs the nearest phosphate (depicted inFIG. 5 by the two yellow hydrogen bonds), while a base reader recognizesits Watson-Crick complement (depicted in FIG. 5 by the three redhydrogen bonds) when it is present. So long as both molecularrecognition events overlap in time, a large current will flow, with theconsequent charge pulse signaling identification of the target base. Inanother embodiment, one electrode is fitted with a base or base readerthat will recognize its Watson-Crick complement and the other electrodeis fitted with a base-pair reader, described herein below.

FIG. 5 a shows the device 2 of FIG. 5 arranged in an apparatus 100configured to read bases in ssDNA, by measuring tunneling current. Aconstriction in the form of a nanopore 1, is formed in the device 2. Inthe embodiment of FIGS. 5 and 5A, the constriction is in the form of ananopore 1 which passes through a thickness of a substrate. It isunderstood, however, that a constriction may take on other forms andarrangements as well. Thus, in an alternate embodiment, the constrictionmay comprise a narrowed portion of a microfluidic channel formed on asurface of a substrate. In other words, the constriction may lie on topof a device with the target molecule passing from a first chamber on topof the device to a second chamber on top of the same device. In suchcase, the constriction connects the two chambers which are separated bya surface partition. In another embodiment, the constriction comprises ananopore created by a single walled carbon nanotube (SWCNT).

In one embodiment, the device 2 comprises a chip 2 which, in turn,comprises a substrate provided on a thin Si₃N₄ membrane. The top of thechip 2 seen in FIG. 5A comprises a first side while the bottom of thechip 2, which is hidden from view in FIG. 5A, comprises a second side.Thus, the device 2 may be considered to comprise a partition having afirst side and a second side, and the ssDNA translocates from one sideof the partition to the other side of the partition, via theconstriction 1. First and second electrically conductive sensingelectrodes, 3, 4 which are spaced apart from one another by a gap, areprovided adjacent the nanopore 1 on the first side of the chip 2. In oneembodiment, the gap between the first and second electrodes is between1.0-5.0 nm, though larger gaps may be possible. The first and secondsensing electrodes are preferably formed of gold, though they mayinstead be formed of other electrically conductive materials.

The chip 2 is mounted in the device 100 such that the chip's first side(top) is exposed to a first fluid chamber 6 (“upper chamber”) and thechips second side (bottom) is exposed to a second fluid chamber (“lowerchamber”). As seen in FIG. 5A, the second fluid chamber 6 contains thessDNA 9 to be read, while the first fluid chamber receives the ssDNA 9translocating through the nanopore 1 which forms a passage between thetwo fluid chambers 5, 6.

On the first side of the nanopore 1, a first affinity element 8 istethered to the first sensing electrode 3 via a first flexible linker.Generally speaking, a “linker” is a chemical entity designed so as topermit adequate motion of the affinity element to self-assemble on thetarget while remaining in electrical communication with an electrode.The first flexible linker itself may be bonded to the first electrodevia an electrode molecular anchor, such as a thiol. In one embodiment,the first affinity element 8 comprises guanidinium or a guanidiniumderivative such as guanidinoethyl disulfide. Guanidinium performs thefunction of grabbing the phosphate of backbone of the ssDNA 9 and thusserves as “phosphate grabber.”

On the second side of the nanopore, a second affinity element 7 istethered to the second sensing electrode 4 via a second flexible linker.The second flexible linker itself may be bonded to the second electrodevia an electrode attachment molecule, as described above. In oneembodiment, the second affinity element 7 comprises a base reader whichis configured to recognize one of the four bases on the ssDNA 9. Ingeneral, both the phosphate grabber and the base reader form hydrogenbonds that are readily broken at room temperature. Thus, the bondsformed during translocation are made and broken on a timescale thatpermits rapid binding and release of the target while still allowing fordetection and measurement of a tunneling current.

In one embodiment, the flexible linkers associated with either or bothsensing electrodes may comprise an alkane. A thiol serves as theelectrode molecular anchor, and so the combined linker-electrodeattachment molecule may comprise —CH₂—CH₂—SH. The flexible linkers allowthe reader to rotate freely and also allow the bases in the reader torotate and associate with its complement.

In one embodiment of the present invention, reference (RE) and counter(CE) electrodes are incorporated into the chip itself, spaced a fewmicrons from the tunneling gap.

As also seen in the embodiment of FIG. 5A, a first magnetic bead 10 maybe affixed to a leading end of the ssDNA 9 and used to pull the ssDNA 9through the nanopore 1. Optical tracking of the bead allows transit ofthe ssDNA to be followed to within 20 nm. It is understood, however,that a second magnetic bead may be used on the second side of the device(i.e., in second fluid chamber 5) to help untangle the secondarystructure of the DNA.

A pair of polarization electrodes 11 are used to polarize the nanopore 1for electrophoretic transport of the ssDNA 9. A voltage bias 12 and acurrent monitor 13 are used to control the electrophoretic transport.

The first and second sensing electrodes 3, 4 are connected to a sensingelectrode bias 14 and also to current measuring circuitry 15 to gaugethe tunneling current as each nucleotide is detected duringtranslocation of the ssDNA 9. It is from the measured tunneling currentat one or more nanopores that the corresponding portion of the ssDNA canbe identified.

FIG. 20 shows an exemplary arrangement, where the device 381 is placedin a microfluidic channel 382. The stream lines show the fluid beingdiverted in order to pass through the constriction 383. The electrodesurfaces within the microfluidic channel are insulated from the fluid inthe channel by a protective layer of insulation 384. Connections to theelectrodes 385 exit the structure outside of the fluid channel.

FIG. 21A shows an exemplary cross section of an embodiment of a device391 through a fluid channel showing a planar electrode arrangement. 391is the top layer of insulation, 392 is a first metal or dopedsemiconductor layer, 393 is a second layer of insulation, 394 is asecond metal or doped semiconductor layer and 395 is the insulatingsubstrate on which the structure is formed. The size of the constrictionbetween the electrodes is determined by the thickness of the secondlayer of insulation 393. The structure is assembled by planar depositionof alternating conducting and insulating layers on the substrate,followed by form ation of a channel, 396, through the entire structure.

FIG. 21B illustrates one embodiment of dimensions for operation of thedevice seen in FIG. 21A. In this embodiment, the spacing between theelectrode pair, 392 and 394, is L and the diameter of the channel 396diameter is d. In order to obtain adequate tunnel conductance throughthe target molecule, L lies in the range from 0.5 to 10 nm. The channeldiameter is constrained only by the requirement that a molecule enteringthe channel touch the sides, and hence the electrodes, during itstransit through the channel. If the speed of fluid flow through thechannel is V meters per second than the time spent between the electrodepair is:t=V/L seconds.

In this time the molecule must diffuse a lateral distance d, given byd=√Dtwhere D is the diffusion constant of the molecule. Thus the maximumspeed of transit of the sample passed the electrodes is given by:V≦DL/d2.

Thus the speed with which fluid can be processed decreases rapidly asthe constriction size is increased. For example, with L=5 nm, d=10 nmand D=100 (μm)2/s (typical of a small protein), V is preferably lessthan 5 mm/s.

FIG. 22A shows a top view and FIG. 22B shows a cross section of afurther embodiment using opposing electrodes. 401 is a first linearmetal or doped polysilicon electrode. 402 is a second linear metal ordoped polysilicon electrode. 403 is a channel that has been milledthrough the entire structure. 404 is a protective insulating layer thatcovers the electrodes. 405 is the underlying insulating substrate. 406Aand 406B are electrodes, which may be chemically deposited layers ofconducting metal used to achieve a small constriction between theopposing electrodes 401 and 402.

One exemplary manner of assembling the structure of FIGS. 22A and 22B isto make a stripe of doped polysilicon conductor on the substrate 405,then coat over this stripe and the substrate with an insulating layer ofoxide, then use a focused ion beam mill to cut a slot through the entiredevice, separating the stripe of polysilicon into the two opposingelectrodes 401 and 402. The channel size resulting from focused ion beammilling is likely to be about 30 nm, so the constriction is narrowed tothe desired nanometer dimension by, for example, electric chemicalgrowth of a metal such as gold on to the exposed conducting polysiliconelectrodes. This growth can be continued until the junction isshort-circuited, and then a small amount of gold removedelectrochemically, leaving a constriction of the desired size.

FIG. 23 provides an exemplary electrical arrangement embodiment of anapparatus of the present invention. The apparatus comprises a channel412 formed through a chip 411. Two external connections to theelectrodes 413 and 414 are placed on a chip carrier 415 that allowsfluid access to the front and back of the channel 412. One electrode 413is grounded, while the other is connected to the inverting terminal of acurrent to voltage converter, 416. The non-inverting terminal isconnected to a source of bias, 417, so that feedback is applied throughthe current to voltage conversion resistor R, the inverting terminal isheld at a potential of V volts with respect to ground, thus biasing thenon-grounded electrode. The output signal of the current to voltageconverter 416 is −iR volts where i is the tunnel current signalgenerated by detection of one or more molecules in the device. TypicallyV lies in the range from 10 mV to 1V, so that a 1 nS tunnel conductancefor a detected molecule would yield a current between 10 pA and 1 nA.With R=1GΩ this leads to output voltages between 10 mV and 1V. Thesignal to noise of such a detection system may be improved, for example,by using a larger resistor R, but the response time (RCin, where Cin isthe electrode stray capacitance) becomes slower. With an R=1GΩ resistor,the shot noise is a tiny fraction of 1 pA, while a stray electrodecapacitance of 1 pF (possible with appropriate electrode andinstillation design) yields a response time of 1 ms.

In each of the embodiments seen in FIGS. 20-23, it is understood thatthe various linkers, affinity elements, and the like are connected tothe various electrodes to create a functioning apparatus in accordancewith the present invention.

Although the present invention has been described to a certain degree ofparticularity, it should be understood that various alterations andmodifications could be made without departing from the scope of theinvention as hereinafter claimed.

The chip 2 of the device 100 seen in FIG. 5A may be fabricated in anumber of ways.

In one embodiment, a number of such chips 2 may be created at the sametime using a single wafer in conjunction with the following principalsteps:

-   -   Step 1. Grow 100 nm Si₃N₄ on the top side of the Si wafer.    -   Step 2. Photolithographically pattern sensing wires on top of        the Si₃N₄ using lift-off. The sensing wires will later be cut        into two electrodes for each chip.    -   Step 3. Photolithographically pattern a reference electrode (RE)        and a counter electrode (CE) on the underside, the RE and the CE        being brought to the edge of the windows (see, e.g., FIGS. 7 &        9).    -   Step 4. Grow 200 nm SiO_(x) on both top and bottom to insulate        the various electrodes.    -   Step 5. Pattern and cut windows through SiO_(x) and Si with HF        and KOH etches, exposing part of CE and RE on underside.    -   Step 6. On FIB, cut through SiO_(x) and cut sensing wires and        shape ends. The gap between the electrodes is about 20 nm.        Exposed metal area should be less than a few square microns to        minimize leakage current from sensors.    -   Step 7. Turn chip and FIB mill nanopore through Si₃N₄ centered        on gap between electrodes. The thus-formed pore is 5 to 10 nm at        electrodes. Steps 5 and 6 can be done automatically under        computer control for many devices.    -   Step 8. Clean excess Ga ions from FIB milling with nitric acid.    -   Step 9. Place gold plating solution below the chip (Si side) and        salt solution above it (Si₃N₄ side).    -   Step 10. Plate Au onto sensing electrodes until a predetermined        tunnel current is obtained between the two sensing electrodes.        If this is coincident with a drop in the pore ionic current (IP)        then the electrodes are centered. The parameters may be adjusted        so that this process can be automated for production.    -   Step 11. Open gap by stripping Au to achieve optimal size.    -   Step 12. Rinse.    -   Step 13. Functionalize the chips by exposing them to equimolar        mix of phosphate grabber and base reader.

Step 14. If specific functionalization is necessary, hold one electrodeat >−1V Ag/AgCl and load a first recognition reagent comprising thefirst affinity element. Rinse and then expose to the second recognitionreagent which comprises the second affinity element. Rinse again.

-   -   Step 15. Mount chip in device so as to form the lower chamber 5        and upper chamber 6.

It is understood that the wafer may have a large array of suchnanopores. In some embodiments, all the nanopores on a wafer may befunctionalized in the same exact manner. In other embodiments, however,the nanopores on a wafer need not all be functionalized with the sameexact affinity elements.

In one embodiment, the wafer may be considered to comprise an array of2×2 sub-arrays. Each nanopore in a 2×2 subarray may then have aphosphate grabber (such as guanidinium) as the first affinity elementand a different one of the four base readers as the second affinityelement. Or in another embodiment, the first affinity element is a baseand the second affinity element is a base-pair reader. This way, each2×2 subarray comprises all four base readers for use in devicesconfigured for “parallel” DNA sequencing. The wafer may then be cut intochips, each chip having a single 2×2. Alternatively, the wafer may becut into larger chips, each such chip comprising a plurality of such 2×2subarrays. This redundancy on a chip can increase the certainty ofrecognition, as discussed further below.

It is understood that multiple 1×4 sub-arrays may be formed instead of2×2 subarrays. In such case, the wafer may be considered to compriserows of nanopores whose members are similarly functionalized. Forinstance, the wafer may comprise a number of rows that is a multiple offour. Each nanopore in a given row may then have a phosphate grabber (ora base) as the first affinity element, and the same base reader (or basepair reader) as the second affinity element. Four rows that are adjacentto one another, may then have a different base reader as the secondaffinity element in all their nanopores. This allows one to cut up sucha wafer into chips comprising a single 1×4 subarray, or even into largerchips comprising a plurality of such 1×4 subarrays.

Each nanopore 1 is functionalized by its associated second affinityelement 7 to recognize one of the four bases. Therefore, to sequenceDNA, it is understood that either: (a) a single copy of the DNA mustpass through a “gauntlet” comprising four differently functionalizednanopores (“serial read”), or (b) four identical copies of ssDNA mustpass through four distinct, differently functionalized nanopores(“parallel read”).

When a single copy of DNA is used, the nanopores belong to differentchips and the DNA is threaded through the four chips. Readouts of theelectrical current detected from each of the four nanopores can bealigned, using the known rate of translocation and peak current valuessignifying a match to determine the DNA sequence.

When four identical copies of DNA are used, it is desirable that theytranslocate in synchrony. Readouts of the electrical current detectedfrom each of the four nanopores can then be compared to look for peakvalues signifying a match.

Thus, in one embodiment, a device may be used to sequence DNA by thefollowing set of principal steps:

-   -   Step 1. A plurality of such nanopores, each functionalized to        recognize one of the four bases, should be provided. This can be        done using either serial reads or parallel reads, as described        above.    -   Step 2. Place DNA in lower chamber associated with each such        nanopore. Optionally modify the DNA so as to allow entry into        the pore from one direction only. In one embodiment, this may be        done by tethering the DNA to a bead.    -   Step 3. Electrophorese the DNA through the pore. If extra        pulling force is needed, functionalize the end that passes        through pore (after having been modified with e.g., biotin) and        attach magnetic bead.    -   Step 4. Pull DNA through by electrophoresis and/or magnetic        bead.    -   Step 5. Record current pulses (It) as a function of time.    -   Step 6. Align data from a plurality of reads for each type of        base reader.    -   Step 7. Align data from all 4 reads.

The present invention utilizes the principle of hydrogen bonding formolecular recognition. A number of measurements of hydrogen-bondmediated tunneling using various combinations of bases that formWatson-Crick or mismatch hydrogen bonding have demonstrated thefeasibility of this readout.

STM measurements have been made by the present inventors (see FIG. 5 andpages 14-15 of U.S. provisional 61/103,019). Data measurements of tunnelcurrent as a function of distance were performed by the presentinventors (see FIGS. 6-7 and pages 15-16 of U.S. provisional61/103,019). The present inventors have been able to show that theycould distinguish between a G-C and a G-T base pair (one hydrogen bonddifference) using the present invention (see FIGS. 8-10 and pages 16-18of U.S. provisional 61/103,019). The inventors have formed and measuredgaunidinium contacts to DNA (see FIGS. 11-14 and pages 18-20 of U.S.provisional 61/103,019). In addition the inventors have been able toread base compositions from adsorbed DNA (see FIG. 15 and pages 20-21 ofU.S. provisional 61/103,019) and have provided a theoreticalconformation of the obtained experimental results (see FIG. 16 and pages21-22 of U.S. provisional 61/103,019).

The present inventors have manufactured and tested nm wide tunnel gaps(see FIG. 17 and page 24 of U.S. provisional 61/103,019) and tested thegaps for a recognition response (see FIG. 18 and page 24 of U.S.provisional 61/103,019). In some embodiments, is preferred that thattunnel gap is sized to correspond to a conductance minus the bridgingmolecule of 0.012 nS.

The inventors have also optimized electrode functionalization (see pages24-25); optimized electrochemical fabrication of the device (see page25); characterized electrode stability (see page 25); and have shownactive control of the tunnel gap (see FIG. 20 and pages 25-26 of U.S.provisional 61/103,019).

Generation of Molecular Recognition Signals in a Tunnel Gap Fabricatedon a Chip; Optimization of Electrode Design and Fabrication

The problem of how to make ‘molecular alligator clips’ has consumed themolecular electronics community for decades. The problem now appears tohave been solved in flexible junctions (like the STM where one electrodeis controlled with sub precision) but it is extremely difficult forfixed junctions, at least in the case of single molecules. One reason isthat the outermost atoms of each electrode must be in precisely thecorrect position to satisfy the bonding requirements of the moleculethat spans the gap. Some successful experiments have been reported usingelectromigration, a technique in which a nanogap is formed by “blowing”a fuse consisting of nano-scale neck in a wire. But the same techniquehas been shown to mimic molecular electronic effects in the absence ofmolecules owing to the presence of complex metal structures in the gap.The requirements for atomic precision in bonding molecules are mitigatedin the embodiment of FIG. 5 because of the use of flexible linkers aspart of the tunnel junction. This is possible because the embodiment ofFIG. 5 depends on a binary “signal-no signal” output rather than on anabsolute conductance of the gap. Thus, reliable manufacture of “clean”gaps of about 2 to 3 nm in size range and chemical functionalization ofthese gaps for reliable readout, are both realizable.

FIG. 6A-6B shows the results of a different approach to electrode designand the formation of gaps between electrodes. The technology employed tomanufacture and inspect such “clean gaps” has undertaken a significantleap as a result of the work of Marija Drndić at the University ofPennsylvania. By using electrodes placed on very thin membranes (exactlyas required for nanopores) Drndić's group has been able to imagejunctions using transmission electron microscopy (TEM) to atomicprecision. Moreover, Drndić's group has shown that the lack of electronbackscatter in thin-film supports permits very high resolutionelectron-beam ablation of metal structures.

Another approach to electrode design and the manufacture of nanogaps iselectrochemical deposition and stripping. Electrochemical generation ofnano-gaps has been in use for some years but may sometimes beunreliable.

The present invention also takes advantage of a new approach toelectrochemical generation. In one embodiment of the present invention,reference (RE) and counter (CE) electrodes are incorporated into thechip itself, spaced a few microns from the tunneling gap.

FIG. 7A-7D shows a prototype test-bed large-electrode junction (i.e.,large electrodes, small gap) in accordance with one embodiment of thepresent invention. Gold electrodes that are 2 μm wide by 30 nm high arepatterned by lift-off onto a 200 nm thick SiO₂/Si₃N₄ substrate in acruciform pattern. One electrode is a continuous strip that is cut toform the two sensing electrodes (SE1, SE2), which also serve as theworking electrodes for gold deposition and stripping. Two otherelectrodes (RE, CE) are separated from the central wire by gaps of 3 μmand they serve as built-in counter- and reference-electrodes. Theelectrodes are covered with a 200 nm thick layer of SiO₂. The wafer,containing 25 arrays, each of nine devices, is taken to the focused ionbeam mill (FIB) where a trench is cut across the central wire to formthe two sensing electrodes (SE1 and SE2). The trench is widened andcontinued out to the RE and CE electrodes to form microfluidic channelsin communication with both sensing electrodes and the CE and RE. As aconsequence of the geometry of the ion-beam milling, a 100 nm widetrench at the top of the SiO₂ corresponds to about one to two nm gap inthe gold electrodes. As a result, once the gap is chemically-cleaned ofexcess Ga ions, stable and somewhat reproducible tunnel gaps are formed.

FIG. 8A, which plots the log-current vs. bias voltage data (and thelinear data shown in the inset), confirms the presence of such tunnelgaps. Fits of these data to the Simmons formula yield gap dimensionsthat are on the order of one nm. By placing a drop of gold-platingsolution in the gap and controlling the deposition galvanostatically,while monitoring the tunnel current, I_(t), it is possible to close thegap. Once closed (as detected by current between SE1 and SE2) controlledstripping opens the gap with atomic scale control.

FIG. 5B, which plots conductance as a function of time after the gapsare closed, shows quantum steps in conductance (indicated by arrows)that characterize atom-sized filaments of gold. Thus atomic-scalecontrol of gap size can be achieved by electrochemical deposition andstripping.

Alignment of a Nanogap Electrode Pair with a Nanopore.

Assembly and alignment of the reading head, comprising a pore andelectrodes can be achieved through electrochemical self-assembly ofelectrode pairs. Electrochemical deposition of electrodes minimizes thenumber of one-off nanofabrication steps, resulting in devices that areeasier to manufacture. Furthermore, an electrochemical approach makes itpossible to strip and reuse electrodes, a possible cure for failuremodes related to electrode geometry and functionalization. This alsoreduces costs and enhances reliability.

FIG. 9 presents a strategy for controlled growth of electrodes into thegap, in accordance with one embodiment of the present invention. Thedeposition process is controlled from the trans solution 210 acompartment so that deposition is localized to the region in theimmediate vicinity of the pore 211. In addition to the electrodestarting geometry, other factors affecting this process are reagentconcentrations, electrode potentials and pore geometry. The process iscomplicated by the high resistance of the pore. Simulations and TEMmeasurements can be used to characterize real junctions to optimize theprocess of forming the electrodes. Meller's TEM approach can beadvantageously employed to produce small pores, but it may also bepossible to eliminate the TEM “filling” step. While the electrode gapcan be quite large, the nanopore 211 must be small enough to permit onlyone DNA strand to pass at a time. Therefore, alternatives to TEM couldgreatly simplify the production of reading heads. Recent reports ofcontrolled formation of pore as small as 5 nm by FIB indicate that onemay start with a 20 nm pore cut into a 20 nm constriction to form thetwo sensing electrodes (the starting geometry shown in FIG. 10C). Onecan mill through a thin Si₃N₄ from beneath the electrodes (which arevisible in the dual-beam FIB through the membrane). The gold electrodescan then be electroplated out into the gap, narrowing both the gap andthe pore to the desired size (˜2 to 3 nm). Optimizing pore size can beadvantageous given that smaller pores result in greater DNA-poreinteraction while distinct ssDNA translocations have been observed inrather large pores. In the event that a small (<2 nm) pore is requiredto ensure translocation of only single strands, or to remove secondarystructure, one may start with pores that have been “shrunk” on the TEM.

A computational approach can be used to simulate the electrochemicalprocesses in three stages: (1) 2D modeling of the electrodepositionprocess ignoring double-layer effects. (2) Subsequent inclusion ofdouble layer effects. (3) Finally, a full 3D model including the doublelayer. See FIGS. 21-22 and pages 27-29 of U.S. provisional 61/103,019.

Design and Synthesis of DNA Base-Readers

Elimination or reduction of base-pairing mismatches simplifies therobustness of the sequencing. This can be realized by using morespecific DNA Base-Readers. Better affinity elements based on chemicalprinciples and theoretical modeling help with the design of recognitionreagents. Once synthesized, affinity elements can be readily and rapidlycharacterized using STM methods, such as those described herein.

Each of the four bases has a distinguishable Watson-Crick edge, allowingone to design a reader for each of them. (See FIG. 23 of U.S.provisional 61/103,019). Four separate readers are likely to berequired, although it may also be possible to employ a “universalreader” having a particular structure. A set of candidate structures canbe screened to determine their specificity and immunity to mispairing. ADNA base reader should have the following chemical and structuralfeatures: (1) donor and acceptors sites for formation of stable hydrogenbonds; (2) planar n system capable of stacking interactions andefficient mediation of tunneling; (3) the molecules must be constructedsuch that the Watson-Crick base pairing occurs with high specificity;(4) they could incorporate steric obstruction of mismatches; and, (5)should be stable to oxygen, light, water, and electrochemical reactions,once coupled to the electrodes. It is best to reduce manipulations ofthe target DNA (such as incorporation of modified nucleotides byenzymes) to a minimum in view of the goal of reading long, native DNA.

Electronic structure calculations can be carried out prior to synthesisboth to verify the proposed bonding, and to test the effects of alteringthe structure of the heterocyclic rings on electronic conductance.

The Adenine Reader (A Reader)

A commercially available 5-mercaptouracil may be used as an Adeninereader. 5-mercaptouracil can form a Watson-Crick base pair with adenine,but it can also mispair with other DNA bases C, G, and T (see FIG. 24 ofU.S. provisional 61/103,019). Because each of these mismatched basepairs has a similar hydrogen bonding pattern to the Watson-Crick basepair, it may be very difficult to distinguish them electronically,making adenine the most difficult base to identify unambiguously. Thisproblem is not necessarily fatal if high fidelity data are availablefrom the three other readers, but a selective A-reader is highlydesirable.

FIG. 11 shows a peptide nucleic acid (PNA) trimer comprising onemodified uracil flanked with universal bases, which may be synthesized.Such a PNA may serve as a selective A-reader. PNA is a structural mimicof DNA but it forms more stable DNA duplexes and is more sensitive tomismatches than its DNA counterpart. By using a PNA trimer forrecognition, one may convert a single base pairing process into aDNA-PNA hybridization process. Thus, the base pairing specificity ofmodified uracil relies not only on hydrogen bonding but also on stackingwith its nearest neighbors. The additional stacking interaction promotesthe preorganization of the base reader into the “right” conformation forWatson-Crick base pairing. Universal bases form base pairs with normalDNA bases indiscriminately so the PNA trimer should have no selectivityto the (n−1) and (n+1) flanking bases on the target DNA. It has beendemonstrated that a universal base can enhance the mismatchdiscrimination in the DNA duplex thermodynamically and enzymatically. Inone embodiment of the structure, a propargylthiol linker is attached at5-position of uracil for connection to the electrode. In the event thatthis linker is not long enough, another “molecular wire” may be used.The PNA trimer can thus be tested against a series of adenine centeredDNA trimers with varied base contexts at their two ends on goldsubstrates using the STM method. Such a strategy may also be applied tothe design of other base readers.

The PNA trimer can be synthesized manually or in an automated peptidesynthesizer. The synthesis of universal base PNA monomer has beenreported in the literature The modified uracil PNA monomer may besynthesized starting from 5-iodouracil-1-acetic acid. The startingmaterial reacts with ethyl N-[2-Boc-aminoethyl]glycinate, providing a5-iodouracil PNA monomer that can be converted into the desired productthrough the Sonogashira coupling with 3-benzoylthio-1-propyne followedby treating with di-tbutyl-1-(tbutylthio)hydrazine-1,2-dicarboxylate.

The Cytosine Reader (C Reader)

FIG. 12 illustrates that 8-Mercaptoguanine can serve as a C reader.Guanine in general forms stable mismatched base pairs such as G-G, G-A,and G-T. Ideally, however, these mismatches would be reduced. Sekine andcoworkers have demonstrated that 2-N-acetyl-3-deazaguanine (a2c3G) ismore selective to cytosine than guanine (see FIG. 13), and alsodestabilizes the GA mismatch. Compared to guanine, one of the undesiredhydrogen bond acceptors is removed and the rotation of the NH₂ group isconstrained in a2c3G.

FIG. 13 shows a number of modified guanines which, based on the work ofSekine, may serve as C readers. A common feature of these molecules isthat their Watson-Crick edge remains unchanged and the undesired atomsare left out. S-acetylguanine is an amine-acetylated derivative of8-mercaptoguanine, which can be used to determine how acetylation of theamine affects the specificity of the guanine. With this control,S-deazaguanine, a deaza derivative of 5-acetylguanine (with the3-nitrogen removed) should reduce the sheared G-A mismatch.S-aminopyridone is the simplest candidate C reader and it should havethe highest specificity. A PNA trimer containing 3,7-dideazaguanine(S-deazaG) is potentially useful for this purpose as well.

S-acetylguanine can be synthesized starting from9-methyl-8-mercaptoguanine. The thiol group is first protected in atbutyl disulfide form, and then the starting material treated withacetyl chloride followed by Al—NiCl₂-THF. The synthesis ofS-deazaguanine is straightforward using 3-deaza-9-methyl-guanine as thestarting material. S-aminopyridone can be synthesized starting from4-iododiamonopyridine prepared according to the reported procedure.First, 4-iodo-6-acetylaminopyridone can then be synthesized by adoptingthe method used by Sun et al, and then converted to the desired productby treatment with thiourea. The key step in synthesis of S-dideazaG PNAmonomer is iodonation of dideazaguanine. The approach developed byRamzeva and Seela can be employed for this. If such an approach is foundto have a selectivity problem, one may first prepare 7-iododidazaguanineusing the regioselective reaction controlled by a bulky group at9-position of dideazaguanine and then convert it to the desired product.

The Guanine Reader (G Reader)

In one embodiment, 6-Mercaptocytosine, a cytosine derivative, can beused as a G reader. FIG. 14 shows that 6-Mercaptocytosine should form amore stable base pair with guanine compared to the mismatches in neutralconditions. Other candidates include 5-mercaptocytosine,5-mercapto-1-methylcytosine, 6-mercapto-1-methylcytosine, and1-(2-mercaptoethyl)cytosine, which can easily be synthesized fromcommercially available starting materials. Studies of these moleculesallow one to optimize the G reader attachment and to determine how theN−1 methylation of cytosine affects its specificity. The effects of pHon the recognition of the G reader should also be taken intoconsideration. It is known that protonation on DNA bases enhances thestability of mismatched base pairs. Under slightly acidic conditions,Cytosine forms stable hydrogen bonded base pairs with protonatedcytosine (C+) and adenine (A+). The protonation alters the electronicstructure of DNA base pairs, resulting in changes of their electronicproperties. Thus, pH is a factor in achieving a high specificity. Theelectrode side of the pore may be somewhat basic owing to thepolarization of the pore used to translocate the DNA into the cischamber.

FIG. 15 shows the basepairing of a tricyclic cytosine analogue (called a“G-clamp” with guanine. Lin and Mattecucci have reported that such aG-clamp can simultaneously recognize both Watson-Crick and Hoogsteenedges of a guanine when it was incorporated into DNA (FIG. 15). TheG-clamp has shown a higher specificity than its counterpart,5-methylcytosine. The G-clamp may also be evaluated for its suitabilityas a G reader. A thiolated G clamp can be synthesized based on aprocedure published by Gait and coworkers.

The Thymine Reader (T Reader)

In one embodiment, 2-amino-8-mercaptoadenine, a derivative of2-aminoadenine (DAP), can be used a T reader. FIG. 16 shows that DAPforms a more stable Watson-Crick base pair with thymine due to anadditional N—H ^(. . .) O hydrogen bond. However, it has been reportedthat stability of the DAP-T base is sequence dependent in DNA, which isattributed to varied base-stacking interactions. Thus, a individual DAPcoupled to an electrode should recognize thymines in a single strandedDNA with high selectivity, generating distinguishable electronicsignals. DAP can form mismatched base pairs with C and A. Other types ofmismatches, such as Hoogsteen base pairs, can also occur. As shown inFIG. 16, one may employ three analogues of diaminopurine to improve itsspecificity and affinity to the thymine base.2,6-Diacetamido-4-mercaptopyridine, which can be synthesized by treating2,6-diacetamido-4-iodopyridine with sodium hydrosulfide, is a simple DAPanalogue which is more specific and stable. In general, the DAP-T basepair is less stable than the G-C base pair. Recently, Brown andcoworkers reported an analogue of adenine,7-aminopropargyl-7-deaza-2-aminoadenine, which could form an “A:T” basepair with stability comparable to G:C. One may therefore test the basepairing specificity and stability of its analogues7-deaza-2-aminoadenine and 3,7-dideaza-2-aminoadenine by incorporatingthem into the PNA trimer, respectively. The corresponding PNA monomerscan be synthesized from commercially available starting materials6-Chloro-7-deazaguanine and 4,6-dichloro-1H-pyrrolo[3,2-c]pyridine usingchemistries described above.

A Universal Reader

The present invention provides a universal reader and the use of auniversal base reader in DNA sequencing. The universal base reader iscapable of recognizing the four natural DNA bases with distinguishablesignatures such as 4-(mercaptomethyl)-1H-imidazole-2-carboxamide. Itincludes two hydrogen bonding donors and two hydrogen bonding acceptors,one half on the aromatic imidazole ring and the other half on the amideside group. The molecule can be attached to the electrode through thethiol group.

FIG. 17 shows that the amide group is relatively free to rotate aroundbond a, and the whole imidazolecarboxamide can freely rotate around bondb. In the solution, the molecule exists in a mix of variedconformations. FIG. 17 also illustrates how this molecule base-pairswith each of DNA bases in a different conformation. This universal baseis capable of forming two specific hydrogen bonds with all four basesindiscriminately. See FIG. 17. In the same manner, the universal readercan hydrogen bond with methylcytosine in DNA. Each of the conformationscan be formed by free rotating the sigma bonds, so one may expect thateach base pair has a similar free energy. However, the second universalbase interacts with the universal-DNA base pairs differently (see FIG.56). Thus, it may be possible to read unique signals out of thetunneling device.

The synthesis of 4-(mercaptomethyl)-1H-imidazole-2-carboxamide startsfrom (1-trityl-1H-imidazole-5-yl)methanol. First, the hydroxyl group canbe converted to t-butyldisulfide as a latent thiol function, and then acyano group introduced to 2-position of the imidazole ring, which can behydrolyzed to carboxamide. Finally, the desired product can be obtainedby detritylation and reduction of the disulfide.

The present invention also provides another universal base reader:5-(2-aminoethyl)-1H-imidazole-2-carboxamide. FIG. 55 shows a synthesisscheme for the universal reader5-(2-aminoethyl)-1H-imidazole-2-carboxamide. Starting fromamine-protected histamine (1), the imidazole ring is protected with2-(trimethylsilyl)ethoxy)methyl group (SEM)(2), and then a cyano groupis introduced to its position by treatment with1-cyano-dimethylaminopyridinium bromide (3). Hydrolysis of the cyanogroup with basic hydrogen peroxide and removal of protecting groups willfurnish the desired compound 5-(2-aminoethyl)-1H-imidazole-2-carboxamide(4).

For theory and modeling of base readers, see page 34 of U.S. provisional61/103,019, which is incorporated by reference.

FIG. 56A shows that two universal readers can form triplexes with DNAbases through hydrogen bonding. In these triplexes, the universal readerinteract with DNA bases differently so that each triplex generates adifferent current signal responding to a individual DNA base. Theuniversal base reader can be used in the systems described herein forDNA sequencing in conjunction with a nanopore/carbon nanotube. Theuniversal base reader is tethered to a nanopore and the ion currentthrough the nanopore measured as DNA passes through it, stalling fordelay times characteristic of the strength of binding to each base tothe universal base reader. This readout is analogous to thatdemonstrated for a protein nanopore functionalized with a cyclodextrinadaptor (See Clark et al., Nature Nanotechnology, 2009. 4:265-270).

By using a pair of universal base readers, (one universal base readerattached to one electrode and a second universal base reader attached toa second electrode), preferably in any of the devices or methodsdisclosed herein or in PCT/US09/37563, filed on Mar. 18, 2009, one canread tunnel current across recognition complexes (shown in FIG. 56). Theuniversal base pair reader may be coupled to single walled carbonnanotubes (SWCNT) by means of a primary amine tethered to theheterocylic ring by a short, but flexible ethylene linker. Covalentattachments to the ends of the carbone nanotube is carried out using EDCand sulfo-NHS as activating agents to couple the primary amine to thecarboxylate residues induced by O₂ plasma etching of the SWCNT.

Thus, the universal base reader recognizes all bases and forms anadditional set of hydrogen bonds with the complex of the base andanother universal base reader so that a base can bridge the gap betweentwo electrodes functionalized with the same type of universal basereader.

The present invention also provides universal base readers shown in FIG.57 and FIG. 58 b (thiobenzoic acid). By using a universal base reader,this removes the need for selective functionalization of thenanoelectrodes. In addition, as seen in Example 6, use of a universalbase reader may reduce or remove the need for sequence assembly (example6 shows that at least two bases could be read with only one gap).

Base-Pair Readers

The present invention further provides an embodiment where devices ofthe present invention utilize a first recognition element comprising abase that will recognize and bind to its Watson-Crick complement (i.e.A, G, C or T) and a second recognition element comprising a base-pairreader discussed herein below. This provides a new method for formingchemically-specific chemical contacts so DNA, by forming self-assembledhydrogen bonded contacts across each base pair. It is based on theability of certain reagents to form multiple, selective hydrogen bondedcontacts to pairs of bases. See Doronina, S. O. and J.-P. Behr, Towardsa general triple helix mediated DNA recognition scheme. Chemical SocietyReviews, 1997. 26: p. 63-71; Fox, K. R. and T. Brown, An extra dimensionin nucleic acid sequence recognition. Quarterly Reviews of Biophysics,2005. 38: p. 311-320.

The inventors have determined that absolute tunnel conductance forA-Thymdine, 2AA-Thymidine and G-deoxythymidine and have used this datato understand and design an optimal fixed tunnel gap for readouts of DNAbasepairs. There is very small conductance across an entire DNA moleculeso obtaining electronic sequences of DNA utilizes readout schemes thatinvolve short tunneling paths, such as those obtained with the use ofbase-pair readers (as one recognition element) and a base reader or baseas the other recognition element.

Preferably the base-pair readers are synthesized with a thiol linkage(for attachment to a gold electrode) and an amide linkage (forattachment to CNT). See FIG. 38 for exemplary trans base-pair readers inaction. See also example 2.

This process is illustrated for an oligomer containing a guanine base inFIG. 24. By means of one of the nanopore-electrode schemes describedabove, an oligomer, 1, containing a G base passes by a first electrode 4to which is attached a C base 2. This base forms a triply hydrogenbonded complex with the target G base as shown in 6. This complex thenforms another three hydrogen bonds with the G-C basepair reader 3 thatis attached to an adjacent electrode 5. The bonded complex is shown in7. It is stabilized by a total of six hydrogen bonds and forms a shortand direct tunneling path between the electrodes 4 and 5. The short pathmeans that the tunnel conductance will be many orders of magnitudelarger than for paths that cross the whole DNA, particularly because thebackbone sugar appears to be a significant obstacle to tunneling, incontrast to the aromatic bases. This, in turn, means that therecognition reagents may be tethered to the electrodes with long (e.g.,3 or 4 methylene units) flexible linkers, greatly facilitating assemblyof the reader.

A second advantage of the present invention is that that base-pairreader will work for each combination of the pairs, so long as it isfree to rotate about the bond that holds it to an electrode. Thisrotated arrangement is shown in compound 8 of FIG. 24. In this geometry,a G base, tethered to the electrode 4 forms a Watson-Crick pair with a Ctarget in the oligomer being sequenced. The C-G reader then formsanother three hydrogen bonds with the paired GC to again complete atunnel path, this time reading a C on the target DNA.

A similar arrangement can be used to read A's or T's as shown in FIG.25. Here a target T is shown on the oligomer, 9, and a 2-aminoadenine10, tethered to an electrode, 4, is used to form a triply hydrogenbonded complex with the T, 11. This complex in turn forms another threehydrogen bonds with the A-T reader 12 attached to a second electrode 5.The complex, again stabilized by six hydrogen bonds is shown in 13. Onceagain a short and direct tunneling path is completed, allowing for theincorporation of flexible linkers if needed.

The reading of an A target proceeds the same way, except that theWatson-Crick base pair formed with a T tethered to the electrode 4 willonly form 2 hydrogen bonds with the target A. The same A-T reader, iffree to rotate about its attachment point to the second electrode (5)can again form 3 hydrogen bonds with the complex, which is nowstabilized by 5 hydrogen bonds. This arrangement nonetheless offers ashort and quite strong tunneling path between the electrodes 4 and 5.

As used herein, a DNA base pair reader recognizes DNA base pairs throughhydrogen bonding, which can be used for DNA sequencing. As illustratedin FIG. 24, a specific base pair reader recognizes a particular DNA basepair that is formed by a base in a single stranded DNA and a basereader. The base readers discussed above and the base pair readers havethe following common features: 1) able to form triple hydrogen bondswith bases and base pairs; 2) are structurally complementary to basesand base pairs in terms of size, shape, and hydrogen bonding pattern; 3)able to form thermodynamically stable complexes with bases and basepairs to reduce the entropy loss during the reading process based on thepreorganization principle; 4) are fairly conductive: aromatic, plane,heterocyclic; 5) are synthetically available; and 6) are chemicallystable under physiological conditions.

FIG. 26 illustrates a specific base pair reader recognizing a particularDNA base pair that is formed by a base in a single stranded DNA and abase reader attached to an electrode as the DNA passes through ananopore.

Exemplary A-T and T-A base pair readers include 1,8 napthyridine and1,10-Phenanthroline derivates that are capable of forming triplehydrogen bonds with a DNA base pair of T-A or A-T. A preferred A-T orT-A base pair readers comprises the compound of formula I:

wherein X is an H, alkyl, CH₂, CH₂CH₂, alkene, alkyne, or an aromaticring;Y₁ is C, N or X-A (with the proviso that when Y₁ is X-A, Z₁ is notpresent);Y₂ is C, N or X-A (with the proviso that when Y₂ is X-A, Z₂ is notpresent);Y₃ is C, N or X-A (with the proviso that when Y₃ is X-A, Z₃ is notpresent);Z₁ is O or N;Z₂ is O or N;Z₃ is O or N;R is alkyl or carbonyl; andA is S, Se, N, or carbonyl, with the proviso that when X is H or analkyl, A is not present.

Another preferred A-T or T-A base pair reader is a 1,8-Napthyridinederivative comprising formula II:

wherein is an H, alkyl, CH₂, CH₂CH₂, alkene, alkyne, or an aromaticring;Y₁ is C, N or X-A (with the proviso that when Y₁ is X-A, Z₁ is notpresent);Y₂ is C, N or X-A (with the proviso that when Y₂ is X-A, Z₂ is notpresent);Y₃ is C, N or X-A (with the proviso that when Y₃ is X-A, Z₃ is notpresent);Y₄ is C, N or X-A (with the proviso that when Y₄ is X-A, Z₄ is notpresent);Y₅ is C, N or X-A (with the proviso that when Y₅ is X-A, Z₅ is notpresent);Z₁ is O or N;Z₂ is O or N;Z₃ is O or N;Z₄ is O or N;Z₅ is O or N; andA is S, Se, N, or carbonyl, with the proviso that when X is H or analkyl, A is not present.

Another preferred A-T or T-A base pair reader is a 1,10-Phenanthrolinederivative comprising formula III:

wherein is an H, alkyl, CH₂, CH₂CH₂, alkene, alkyne, or an aromaticring;Y₁ is C, N or X-A (with the proviso that when Y₁ is X-A, Z₁ is notpresent);Y₂ is C, N or X-A (with the proviso that when Y₂ is X-A, Z₂ is notpresent);Y₃ is C, N or X-A (with the proviso that when Y₃ is X-A, Z₃ is notpresent);Y₄ is C, N or X-A (with the proviso that when Y₄ is X-A, Z₄ is notpresent);Y₅ is C, N or X-A (with the proviso that when Y₅ is X-A, Z₅ is notpresent);Z₁ is O or N;Z₂ is O or N;Z₃ is O or N;Z₄ is O or N;Z₅ is O or N; andA is S, Se, N, or carbonyl, with the proviso that when X is H or analkyl, A is not present.

Another preferred AT or T-A base pair reader is a 1,10-Phenanthrolinederivative compound comprising formula IV:

wherein is an H, alkyl, CH₂, CH₂CH₂, alkene, alkyne, or an aromaticring;Y₁ is C, N or X-A (with the proviso that when Y₁ is X-A, Z₁ is notpresent);Y₂ is C, N or X-A (with the proviso that when Y₂ is X-A, Z₂ is notpresent);Y₃ is C, N or X-A (with the proviso that when Y₃ is X-A, Z₃ is notpresent);Y₄ is C, N or X-A (with the proviso that when Y₄ is X-A, Z₄ is notpresent);Y₅ is C, N or X-A (with the proviso that when Y₅ is X-A, Z₅ is notpresent);Z₁ is O or N;Z₂ is O or N;Z₃ is O or N;Z₄ is O or N;Z₅ is O or N; andA is S, Se, N, or carbonyl, with the proviso that when X is H or analkyl, A is not present.

Preferred G-C or C-G base pair readers include cinnoline derivativesthat are capable of forming triple hydrogen bonds with a G-C or C-G DNAbase pair. A preferred G-C or C-G base pair reader is a compoundcomprising formula V:

wherein X is an H, alkyl, CH₂, CH₂CH₂, alkene, alkyne, or an aromaticring;Y₁ is C, N or X-A (with the proviso that when Y₁ is X-A, Z₁ is notpresent);Y₂ is C, N or X-A (with the proviso that when Y₂ is X-A, Z₂ is notpresent);Y₃ is C, N or X-A (with the proviso that when Y₃ is X-A, Z₃ is notpresent);Z₁ is O or N;Z₂ is O or N;Z₃ is O or N; andA is S, Se, N, or carbonyl, with the proviso that when X is H or analkyl, A is not present.

Another preferred G-C or C-G base reader is a cinnoline derivativecompound that comprising formula VI:

wherein X is an H, alkyl, CH₂, CH₂CH₂, alkene, alkyne, or an aromaticring;Y₁ is C, N or X-A (with the proviso that when Y₁ is X-A, Z₁ is notpresent);Y₂ is C, N or X-A (with the proviso that when Y₂ is X-A, Z₂ is notpresent);Y₃ is C, N or X-A (with the proviso that when Y₃ is X-A, Z₃ is notpresent);Z₁ is O or N;Z₂ is O or N;Z₃ is O or N;R is alkyl or carbonyl; andA is S, Se, N, or carbonyl, with the proviso that when X is H or analkyl, A is not present.

Additional readers include the compound of the following formula:

wherein R₁ can be H, CH₃, iso-propyl, tert-butyl, halogen, or any bulkygroups such as, but not limited to, carborane, pyrene, adamantane, andfullerene. R₂ can be a short alkyl chain, or any conductive molecularfragments, such as, but not limited to, alkene linkages andphynylene-vinylidine linkages. Each of them contains a functional groupthat can be used for attachment to electrodes, such as amine and thiol,at its end.

Preferably the trans-base pair reader is synthesized with an amide andthiol linker (see example 2) for ease in attachment to the CNT andelectrodes, respectively.

Characterization and Control of DNA translocation through afunctionalized pore

The operation of the sequencer depends on the speed and controllabilityof translocation, the role of the sequence itself in pore-friction andthe degree to which secondary structures delay transit. The many studiesof DNA translocation through a nanopore have generally focused onunfunctionalized nanopores (the exception is Astier et al.).Translocation through a functionalized nanopore is different. Thisshould be evident given that λ-DNA translocates a approximately 6 nmdiameter pore in a few ms (at V=50 mV and 1M KCl), equivalent to a speedof 8 mm/s. It is has been measured that the H-bond lifetime is on theorder of a few ms, which corresponds to a “speed” of just microns persecond, on the assumption that each base is trapped in the reader for amillisecond or so, since even with negative base reads, thephosphate-guanidinium trapping still occurs. The force generated in theSTM pull is probably dominated by the softer material in the gap, asdisclosed in He et al., but it is surely quite large, as H-bonds requireforces on the order of 100 pN to rupture at these pulling speeds (seeFIG. 13 of U.S. provisional 61/103,019) and Ashcroft et al.)). In onestudy of (cyclodextrin) functionalized nanopores, nucleotides becametrapped for significant times, illustrating the large effect of porefunctionalization.

In accordance with one embodiment of the present invention,translocation of DNA through functionalized nanopores can beaccomplished using magnetic beads affixed to a leading end of the DNA asthe primary manipulation tool, because this technology is compatiblewith parallel operation of many reading heads. This is because one setof magnets can pull many beads. The force on a bead of volume v andmagnetization m in a field gradient is given by:

${\frac{\partial B}{\partial z}\mspace{14mu}{is}\mspace{14mu}{given}\mspace{14mu}{by}\mspace{14mu} F_{z}} = {{mv}\;{\frac{\partial B}{\partial z}.}}$

With a field gradient of 100 T/m (readily obtained with permanentmagnets) and 3 μm superparamagnetic beads available from Magsense (WestLafayette, Ind.), forces of up to 150 pN are obtainable. This iscomparable to “optical tweezers” and also similar to the largerelectrophoretic forces experienced in nanopores. A “magnetic tweezers”apparatus having a high field gradient magnet stack can be used to studytranslocation in functionalized nanopores.

FIG. 18 illustrates a device 320 that can track DNA transit to within 10nm by fitting the Airy-fringe pattern around the bead when the objectiveis out of focus. The time resolution is limited to the 50 Hzframe-grabbing rate of the camera interface, but this is adequate with 1ms transit times because the height resolution is limited to around 20bases (and 20 bases can transit in about 20 ms, which is 1/50th of asecond). The device 320 includes a light source 331 which projects alight beam past magnets 332 towards a DNA-tethered bead 333. A lens 334amplifies the bead signal and the resulting image is directed to acamera 335 via a mirror 336. It is understood that other detectionarrangements may also be employed.

FIG. 19A illustrates some of the factors controlling translocation. Theelectrophoretic force, F_(elec), is opposed by H-bond friction 331 inthe gap 332. The entry of the DNA 333 into the pore 334 is typicallyopposed by entropy fluctuations, and, more importantly, secondarystructure 335. For a secondary structure undergoing random thermalopenings at a rate K₀, the opening rate on application of a force f is

${k(f)} = {k_{0}{\exp\left\lbrack \frac{f \cdot x_{ts}}{k_{B}T} \right\rbrack}}$where x_(ts) is the distance to the transition state from the foldedstate along the direction in which the force is applied. The smallestvalues of K₀ for hairpins trapped in a nanopore is about 1 s⁻¹ which isreally very slow. Based on measured values for x_(ts) for a tightmolecular nanopore 50 (about 0.1 nm) an electrophoretic force of 100 pNwould increase the opening rate to about 10 s⁻¹. Thus, secondarystructure could be a significant obstacle to fast reads. The ssDNA couldbe pre-stretched using the magnetic bead 336 but this would reduce thenet force across the pore 334, increasing the rate of backwardsslippage.

As seen in FIG. 19B, pre-stretching would probably require a bead 335 a,335 b trapped at both ends to form a rotaxane with the nanopore.

As depicted in FIG. 19B, in yet another arrangement, one may want toaugment the electrophoretic force using a magnetic bead 335 c.

In each of these experiments, one can measure the output of the sensingelectrodes using a DNA molecule of known sequence. This allows one tocorrelate features in the gross transport (as measured by bead movementand pore current) with local features (as measured by the molecularrecognition signal from the sensing electrodes). One approach is to usethe M13 genome as a source of long ssDNA (6.5 kb). Cutting it requireshybridization with a short helper strand in order to form a local dsDNAtemplate for a restriction enzyme. The short strand is easily removed byfiltration after denaturation. Next, splint-ligation may be used at bothends, putting in a biotin at one end and a digoxigenin at the other,with a two step affinity column purification of the long product.Modification of λ-DNA using incorporation of modified dNTPs followed bymagnetic extraction of the desired strand at high pH may also beperformed. The “flossing” experiment (FIG. 19B) can be carried out bytrapping the DNA-antiDIG bead from the cis chamber using electrophoresisand then functionalizing the DNA in the trans chamber with astreptavidin coated magnetic bead. Finally, a novel “unstructured” DNAmay be available for use that forms Watson-Crick basepairs with naturalbases, but the modified bases will not pair with each other. Presently,the modified nucleotides can be incorporated in runs of up to 600 bases.

It should be evident to one skilled in the art that the foregoingenables one to improve upon the basis design and methodology. Moreparticularly, one may: 1) measure the transit time of known oligomersthrough nanoelectrode pores; 2) re-measure transit times withfunctionalized pores. One can thus test to see if the asymmetry of thebackbone (5′-3′ vs. 3′-5′) affects readout fidelity and transit times,using bead functionalization at one end or the other; or 3) measuretransit times as a function of pH. Secondary structure is removed at lowpH but the same conditions that remove secondary structure (pH>11.6) mayalso destroy H-bonding.

One may measure transit times through both functionalized andunfunctionalized pores with unstructured DNA to measure the extent towhich secondary structure slows entry into the pore. These measurementscan determine the relative contributions of secondary structure andH-bond friction in slowing transit. Magnetic bead experiments may bedesigned to speed up or slow down the translocation as needed. One maytest these arrangements using the functionalized, linearized M13 DNA,correlating the local sequence data from the sensing electrodes with theprogress of translocation as measured optically.

There are multiple forces acting on the ssDNA as it translocates—themagnetic force on the bead, the electrophoresis force on the chargedssDNA, the hydrogen bonding force of the guanidinium attempting to holdthe DNA in place, the hydrogen bonding force of the base-reader on thetarget base, interactions of ssDNA with itself (secondary structure),the viscous force of the water on the magnetic bead, and interactions ofthe DNA with water and with the walls of the nanopore. The length of thetether molecules is also important. Varying it, even slightly, maychange the number of contacts and/or the probability of simultaneousphosphate and base recognition.

Characterization of Signals from Oligomers and Genomic DNA Using a Setof Single Pores.

The sequence-reconstruction problem has two inputs. One is the opticaltracking of transport which could give data at a resolution that couldbe as high as 20 bases. The second is the signals from the molecularreading heads themselves. Reading head data of adequate quality couldpermit alignment of data from all four reading heads with no otherinput. Data for each individual base that is 99.99% accurate may beobtained by a combination of improved affinity elements and multiplereads of the same sequences. If the data from each head are of adequatequality, one may record repeated runs for each type of base with highfidelity.

When sequencing four copies of ssDNA using four nanopores, each nanoporehaving a different base reader as the second affinity element, fourcomponent sequence reads are created. Each sequence read identifies, asa function of base location, the points at which a nucleotide of aparticular type has been detected. Since there may be differences in therates at which the four copies of the ssDNA electrophorese through theirrespective nanopores, there may be an issue of aligning the fourcomponent sequence reads to arrive at a final sequence read representingthe sequenced ssDNA. Blocks of a repeated base (e.g., 4× or 5×) are rareenough that they can serve as good indices of position in the genome,and yet frequent enough so that a significant number of them occur ineach read. Thus, upon obtaining a sequence of component reads of ssDNAfrom each of four readers, one may align the four sequences of componentreads based on one or more preselected blocks of a repeated nucleotide(which hopefully will be present in at least one of the sequences ofcomponent reads.) For example, positive reads of an A5 tract (A-A-A-A-A)would be aligned with unique (or rare) gaps of null readings of 5 basesin extent from the C, G and T readers. This is called the “framingproblem” in parallel transmission of digital data over noisy channels.The problem is greatly simplified if the direction of the data stream isfixed. Thus one can develop protocols for preprocessing input DNA andligating beads (or even just form crosslinked dsDNA blockers) to controlthe entry direction. Once any such needed alignment has been done, onemay then create a final sequence of reads representing the sequencedssDNA from the four component sequences of reads.

The optical tracking data can record each translocation to within 20bases at best, with maybe substantially poorer resolution when entropyand secondary structure fluctuations are taken into account. But it alsoserves as a check on the local alignment algorithms, eliminating grossmistakes (i.e., juxtaposition errors greater than the optical trackingresolution).

Quantitative data obtained from using the device of the presentinvention may be used to develop data analysis tools for rapid sequencerecovery. Some of the issues that can be addressed by such quantitativedata include: (1) the transit times per base in the read (base+phosphateH-bonds) vs. the no-read (phosphate H-bonds only); (2) the frequencywith which a nucleotide is missed altogether; (3) the fluctuations inaverage read speeds; (4) the role of secondary structure; and (5)whether it would help if “stalling,” owing to secondary structure,occurred predictably.

As discussed herein, one may construct a fixed-gap nanopore sequencedevice capable of reading single bases with high fidelity. Such a devicemay incorporate one or more of the following features: electrochemicallygrown self-aligning electrodes, active gap adjustment, and gold as theelectrode material. In use, such a device may be able to deal with thepotential problems of secondary and tertiary structures in long DNAtransits. Furthermore, the assembly of such devices may be facilitatedand even automated for consistency from unit to unit, thereby mitigatinguncertainties in the performance of one-off designs. The assembly andfunctionalization methods allow for reforming and healing of deviceswhose readers have been damaged or otherwise spent.

Another Device of the Present Invention

The present invention further provides a DNA sequencing deviceconfigured to identify at least one portion of a target DNA molecule.The device comprises a partition having a first side and a second side,and at least one constriction passing between the first and secondsides, the constriction being shaped and sized to permit translocationof only a single copy of the DNA target molecule therethrough. Thedevice further comprises a first and second sensing electrodesassociated with the first side of the partition and being separated fromone another by a first gap. The device comprises a first affinityelement comprising a base reader (as described above) connected to thefirst electrode; and a second affinity element comprising a base pairreader (as described above) connected to the second electrode. When aparticular portion of the DNA target molecule translocates through theconstriction, an electrical current passes through the first electrode,the base reader, said particular portion of the DNA molecule, the basepair reader, and the second electrode.

Preferably, the electrical current comprises a tunneling current thatpasses through: a first set of hydrogen bonds established between thebase reader and a nucleotide base of the DNA target molecule, and asecond set of hydrogen bonds established between the base pair readerand the nucleotide base of the DNA target molecule and the base reader.

The present invention further provides a method of sequencing DNA as theDNA molecule translocates through a constriction. The method comprisesproviding an apparatus comprising a DNA sequencing device as describedabove. The device is located in the apparatus such that a first chamberis located on the first side of the device and a second chamber islocated on the second side of the device. A DNA molecule (preferablysingle stranded) is introduced into the second chamber. The DNA moleculeis electrophoresed so that it translocates through the constriction. Theelectrical current passing through the first electrode, the base reader,a nucleotide base of the DNA molecule, the base pair reader and thesecond affinity element, and the second electrode is detected.

The present invention also provides a method of sequencing ssDNAcomprising providing an apparatus having the DNA sequencing device asdescribed above arranged such that a first chamber is located on thefirst side of the DNA sequencing device and a second chamber is locatedon the second side of the DNA sequencing device. At least four copies ofthe ssDNA to be sequenced are introduced into the second chamber. Thefour ssDNA copies are simultaneously electrophoresed the so that eachcopy translocates through one of the four constrictions of the DNAsequencing device. During electrophoreses, at each constriction, it isdetermined whether a nucleotide has been recognized based on anelectrical current passing through the first electrode, the base reader,the nucleotide, the base pair reader and the second electrode, tothereby create four component read sequences, one component readsequence for each constriction.

The present inventors have shown that electrode tethered guanidium ionscan be used to complete an electrical circuit for reading bases in asingle stranded DNA (see example 1, pages 47-49 of U.S. provisional61/103,019). The inventors have also found that a readable signal isobtainable if the circuit is completed using a complementary DNA as asecond connector (see example 2, pages 49-51 and FIGS. 34 and 37 of U.S.provisional 61/103,019).

In methods of the present invention, it may be preferable to perform thereaction in an aprotic solvent (an organic solvent without freeproteins). For example, one universal reader, thiobenzoic acid, can actas a hydrogen bond donor and acceptor in aprotic solvents, since thecarboxylate residue remains protonated. An exemplary solvent istrichlorobenzene. The OH group acts rather like a hydrogen bond donorand the doubly bonded oxygen is similar to a hydrogen bond acceptor sothat these molecules are capable of quite strong pairing with eachother. (Jeffrey, G. A., An Introduction to Hydrogen Bonding. 1997,Oxford: Oxford University Press).

Device Utilizing a SWCNT Comprising Small Gaps Functionalized withRecognition Elements.

SWCNTs a nanopores and as integrated electrodes off the benefit ofproviding a route to control translocation of DNA, especially if the DNAin the tube is essentially immobile at low electric fields. Using SWCNTSas integrated electrodes has the advantage in that the CNT serves anelectrode already aligned with the nanopore. They also provide specificand stable sites for chemical linkages (i.e. for linking recognitionelements) and they provide multiple sites surrounding the nanopore.

Serial Recognition Sequencer

The present invention provides a device comprising a SWCNT forcharacterizing a biopolymer. The device comprises a carbon nanotubehaving an interior channel through which the biopolymer is driven byelectrophoresis. The carbon nanotube comprises a plurality of gaps cutinto the carbon nanotube. The plurality of gaps are functionalized toprovide a characteristic signal at each of the gaps. In a preferredembodiment, the biopolymer is a nucleic acid such as DNA or RNA, and thecarbon nanotube comprises four gaps cut into the carbon nanotube. Inthis embodiment each gap is functionalized with a different nucleosidereader.

With reference to FIG. 27, the present method further provides a devicefor determining the sequence of a nucleic acid comprising: a) a solidsurface to support a carbon nanotube (20) having an interior channelthrough which the nucleic acid can travel; and b) an insulating filmlayer disposed on top of the carbon nanotube.

Preferably the solid surface is an oxidized silicon wafer upon which thecarbon nanotube can be grown. The insulating film layer is a materialthat provides insulation against an electrical current, such aspolymethyl methacrylate (PMMA).

The insulating film layer comprises an origination (21), a first (23), asecond (24), a third (25), a fourth (26) and a termination (22) wellcapable of containing an electrolyte. As shown in FIG. 27, theorigination well (21) is proximal to the first, second, third, fourthand termination wells, and the termination well (22) is distal to thefirst, second, third, fourth and origination wells.

The carbon nanotube comprises a first (27), second (28), third (29) andfourth (30) gap cut into the carbon nanotube. The first gap (27)contacts the first well (23), the second gap (28) contacts the secondwell (24), the third gap (29) contacts the third well (25) and thefourth gap (30) contacts the fourth well (26).

Each of the first, second, third and fourth gaps of the carbon nanotubehas a first gap end and a second gap end. Each of the first gap ends isfunctionalized with a first recognition element (e.g. in FIG. 27 (“PG”))and each of the second gap ends is functionalized with a secondrecognition element (e.g. in FIG. 27 (“AR,” “TR,” “CR” or “GR”)).

The carbon nanotube has an origination end (OR) in contact with theorigination well (21) and a termination end (TE) in contact with thetermination well (22). An origination electrode (32) contacts theorigination well (21) and a termination electrode (33) contacts thetermination well (22). The origination and termination electrodes do notcontact the carbon nanotube.

As used herein the term “contact” means directly contacting or alsomeans being contacted to. Thus the electrode could directly contact thewell or the electrode may be contacted to the well via a salt bridge,for example. Similarly the electrodes contacting the carbon nanotube(discussed below) may directly contact the carbon nanotube or may becontacted to the carbon nanotube indirectly.

The device further comprises a first electrode (42) contacting thecarbon nanotube between the origination well (21) and the first well(23); a second electrode (43) contacting the carbon nanotube between thefirst (23) and the second well (24); a third electrode (44) contactingthe carbon nanotube between the second (24) and third (25) well; afourth electrode (45) contacting the carbon nanotube between the third(25) and fourth (26) well; and a fifth electrode (46) contacting thecarbon nanotube located between the fourth (26) and termination well(22).

The gaps cut into the carbon nanotube are small and are preferably onlya few nm in width.

A voltage bias V_(t) (31) may be applied between the origination well(21) and the termination well (22) by the origination electrode (32) andthe termination electrode (33) to produce a positive bias in thetermination well with respect to the origination well to allowtranslocation of DNA along the length of the carbon nanotube through thecarbon nanotube channel from the origination well through the first,second, third, and fourth wells to the termination well.

So long as the geometry in each well is similar, a voltage drop ofV_(t)/4 appears at each of the four gaps in the absence of any otherconnection. If the termination well (22) is biased positive with respectto the origination well (21) (by a voltage between 0.5 and 2V) DNA willtranslocate along the length of the nanotube (20), from well (21) towell (22) (through wells 23, 24, 25 and 26).

Each part of the carbon nanotube is, in turn, connected by an electrode.As mentioned above, referring to FIG. 27, the device comprises a firstelectrode (42) contacting the carbon nanotube between the originationwell (21) and the first well (23); a second electrode (43) contactingthe carbon nanotube between the first (23) and the second well (24); athird electrode (44) contacting the carbon nanotube between the second(24) and third (25) well; a fourth electrode (45) contacting the carbonnanotube between the third (25) and fourth (26) well; and a fifthelectrode (46) contacting the carbon nanotube located between the fourth(26) and termination well (22). Because of the low resistance of theseconnections compared to the electrical path through the electrolyte, thefour voltage sources, (34, 35, 36 and 37) will set the bias across thegaps. Since the required bias across a gap required for translocation(0.2 to 1V) is quite compatible with the bias required for tunnelcurrent readout (0.2 to 0.6V) the gaps can be biased close to therequired voltage drops for translocation while the overall bias (31)adds an additional drop at each end to drive DNA translocation from theorigination well (21) and out of well (26) into the termination well(22).

Each of the gaps has a first and second end, wherein each of the firstends of the gaps is functionalized with a first recognition elementcoupled to the carbon nanotube and is optionally coupled via a flexiblelinker, such as an amine terminated flexible hydrocarbon linker (e.g. anamine terminated (—CH₂—)₂ flexible linkers). In one embodiment the firstrecognition element comprises a phosphate grabber (PG) (gaunidinium)that has the ability to recognize and form a hydrogen bond with thebackbone phosphate of the nucleic acid. See PCT/US08/59602 (incorporatedherein by reference). The second ends of each of the gaps arefunctionalized with a second recognition element coupled to carbonnanotube. Preferably when the polymer to be sequenced is DNA, the secondrecognition elements are an adenine reader (AR), a thymine reader (TR),a cytosine reader (CR) and a guanine reader (GR). See PCT/US08/59602(incorporated herein by reference) for exemplary base readers. Inaddition, in certain embodiments, the base readers are connected to thecarbon nanotube via an amine terminated flexible hydrocarbon linker(e.g. an amine terminated (—CH₂—)₂ flexible linkers).

In another embodiment, the first recognition element comprises a basethat will recognize its Watson-Crick complement on the DNA and thesecond recognition element comprise a base-pair reader that recognizeseither an A-T or a G-C base pair.

Referring again to FIG. 27, a current output signal (38, 39, 40 and 41)is generated each time a base is recognized by its reader. The presentinvention has the advantage that all four reads, I_(A), I_(T), I_(C) andI_(G) come from the same molecule. Thus, a mixture of molecules, such assheared fragments of a genome, can be presented to the device withoutconfusion caused by the need to identify the signal for each base with aparticular fragment.

The assembly of one of the devices of the present invention is shown inFIGS. 7 to 12. FIG. 28 shows a single carbon nanotube (20) grown on asilicon wafer (50) by means well known to those skilled in the art.Using conventional lithography or metal deposition through a mask,electrodes are then deposited as shown in FIG. 29. Electrodes (42, 43,44, 45 and 46) contact the carbon nanotube, and should ideally be madefrom a metal such as palladium that makes good electrical contacts tonanotubes.

Electrodes (32 and 33) are used to contact the electrolytes in the fluidwells, and can be made from any noble metal. Notably, electrodes (32 and33) do not contact the carbon nanotube. Fabrication of the small gapsalong the nanotube is illustrated in FIG. 30. A thin layer of PMMAresist is spun over the nanotube and the regions to be cut, (61, 62, 63and 64) defined by e-beam lithography. The PMMA is removed in theexposed regions and an oxygen plasma used to make the four cuts shown.As an alternative, the cuts can be made directly with an electron beamin the presence of water vapor. The next step is shown in FIG. 31. Theoriginal layer of resist is removed and a new layer deposited, and sixwells, (71, 72, 73, 74, 75 and 76) opened up by conventionallithography. The ends of the carbon nanotubes at the four cuts (27through 30 in FIG. 27) are carboxylated by the exposure to the oxygenplasma (or water vapor), and so can be chemically modified using amineterminated reagents with an EDC catalyst to form amide bonds. FIG. 32shows the process for modifying the left electrode in each cut with the“phosphate grabber.” This is an amine terminated, ethylene linkedguanidinium, which is positively charged in pH 7 buffer. Thus, when theamine terminated reagent is flowed into each of the four wellscontaining the cut tube, with the junctions biased so that the leftelectrode is biased negative with respect to the right, the positivelycharged reagent couples only to the left electrode. This biasing isachieved with a bias V₁ applied across electrodes (42 and 43), a bias V₂(>V₁) applied at electrode (44), a bias V₃ (>V₂) applied at electrode(45) and a bias V₄ (>V₃) applied at electrode (46). The four fluidreservoirs that feed in the reagents to the four wells must beelectrically isolated from each other to maintain this biasingarrangement. As an alternative, each of the four wells may be separatelyfunctionalized by biasing each electrode pair in turn, and flowingreagents into only the biased well. Such a process would be a littlemore cumbersome, but would eliminate the risk of the positively chargedguanidinium being swept into the chamber with the most negativelycharged electrode (though this is unlikely given the relatively slowtranslocation rates through small nanotube). Once a sufficient time forthe reaction has passed (1 to 5 minutes), the reagents may be flushedout of the wells and the bias removed. This process leaves only the leftelectrode at each cut functionalized. The next step is to expose eachwell in turn to the appropriate reagents, as illustrated in FIG. 33.Here the first well is exposed to the adenine reader (AR)+EDC, thesecond well to the thymine reader (TR)+EDC, the third well to thecytosine reader (CR)+EDC and the final well to the guanine reader(GR)+EDC. After the reaction, the wells may be flushed and the device isready for operation.

Repeated reads may be needed to establish the sequence to the desiredlevel of accuracy. In the case of a mixture of input molecules such asmight arise from sheared genomic DNA, this would require alignment ofthe different reads based on sequence overlap. This would bestraightforward to do for the longer runs (i.e., runs of many kilobases)even in the presence of noise. The present invention has the advantagethat, for each fragment that passes through the device, the mismatchsignals can be used to verify a base call. Thus a positive signal at onereader, accompanied by three mismatch signals at the other three wouldallow for assignment of a base read with much higher confidence thatcould be obtained from a single reader. Such “super assignments” wouldthen aid in the assembly of the final sequence, even for input samplesthat are a random selection of fragments of genomic DNA.

Control of DNA Translocation

In a preferred embodiment the SWCNT are 2 nm in diameter. The presentinventors have found that in tubes this size, the DNA gets “stuck,”requiring some kind of cooperative process for passage. It appears thatthe DNA enters the tube and “piles up” in the tube until some event orpassage of time, the DNA is released from the end of the tube. Thus, theinventors contemplate “freezing” the DNA in place in the tube, byremoving the electrophoretic driving field (temporarily suspending) oncethe DNA has entered the tube. Translocation is then restored byreapplying an appropriate field. Reversing the field might even allowthe same molecule to be read many times by “flossing” it back and forthin the SWCNT. See FIG. 43 that shows one embodiment of the inventionshowing feedback circuitry comprising a current sensing device and atranslocation control signal. See also example 5.

The present invention also provides a method of sequencing DNA usingdevices of the present invention by providing a ssDNA that will betranslocated through the nanopore or CNT. As the DNA travels through thepore or the CNT, it will reach the recognition elements attached to theCNT or to other areas of the device as described above. In a preferredembodiment, one recognition element is a base or base reader that willrecognize and bind (interact) with its Watson-Crick complement. Theother recognition element is a base pair reader that will recognize andinteract (through hydrogen bonding) with either an A-T or a G-C basepair. When the DNA coming through the pore or CNT has the base thatallows for both recognition elements to bond to the DNA, a closedcircuit between the electrodes will occur and a signal is generated. Forexample, if one of the recognition elements is a T base and the other isan A-T base pair reader, when the DNA strand coming through the pore orCNT has an “A” there will be a bond with the A of the DNA and the T onone of the electrodes and another bond between the resulting A-T basepair and the A-T base reader on the other electrodes. As discussedabove, 4 different readers could be run serially, each having an A, G, Cor T base as one recognition element and a corresponding A-T or G-C basepair reader or in other embodiments, the DNA runs through a serialreader having 4 gaps, each gap functionalized with a different base.

Single Molecule Characterization

The present inventors have developed a measurement technique for singlemolecule characterization called the “telegraph noise” technique. Seeexample 4. The present inventors have designed a readout system thatuses a shorter tunneling path rather than measuring conductance of ajunction spanning an entire DNA molecule (i.e. use of base readers andbase-pair readers as two recognition elements tethered to electrodes)and have used this to determine the conductance of individual molecularpairs. The inventors have exploited the stochastic switching ofhydrogen-bonded DNA base-nucleoside pairs trapped in a tunnel junctionto determine the conductance of individual molecular pairs. Thisconductance is found to be sensitive to the geometry of the junction,but a subset of the data appears to come from unstrained molecularpairs. The conductances determined from these pairs are within a factorof two of the predictions of density functional calculations. Theexperimental data reproduces the counter-intuitive theoreticalprediction that guanine-deoxycytidine pairs (3H-bonds) have a smallerconductance than adenine-thymine pairs (2H-bonds). Analysis of thelifetimes of the complexes indicates a change in the type ofbond-breaking as the tunnel gap is made smaller.

Previously, the present inventors and others have shown that electrontunneling can be used as a sensitive local probe to identify individualbases in DNA, a possible basis for direct electronic sequencing. Theinventors have found that current-distance curves collected fromjunctions in which bases attached to a probe are hydrogen bonded tonucleosides or even to intact DNA on an electrode surface faithfullyreport the base composition of the target. A careful analysis of thesesignals shows that they do not arise from single-molecule interactions.Rather, the overall conductance of the tunnel gap is probably set bythrough-space tunneling across a large-area junction containing several(two to ten) molecular pairs. Conducting-AFM measurements provide arough estimate of the conductance of base-nucleoside pairs as lyingsomewhere near 100 to 300 pS. Can the conductances of singlebase-nuleoside pairs be measured directly and accurately? Breakjunctiontechniques for measurement of single molecule conductance are difficultto apply to a system as complex as a base-nucleoside hydrogen bondedpair and they do not report the conductance as measured in the type offixed junction that would be required for DNA sequencing. Stochasticswitching of bonds between molecules and the electrodes of a tunneljunction offers another approach to measuring single moleculeconductance. Ramachandran et al. demonstrated that, for tunnel junctionsusing gold electrodes with thiol-attachment chemistry, fluctuations inthe molecule-metal contact result in the stochastic-switching of STMimages of molecules embedded in a monolayer. Fluctuations in the C—Sibond were monitored in an STM while the transient binding and unbindingof a carboxylate-EDC complex was monitored via conductance fluctuationsin a carbon nanotube. Haiss et al. showed that the time course ofbond-fluctuations in a gold-thiol-molecule-thiol-gold tunnel junctioncould be followed by the simple expedient of placing a gold STM probeabove a gold surface functionalized with bis-thiolated alkane molecules.As the molecules spanning the gap bound and unbound to make and breakthe junction, the tunnel current showed a characteristic fluctuationbetween two-levels. The two-level nature of the signal is a strongindication that a single molecule is trapped in the gap, as switching ofmultiple molecules would generate more complex signals. Thesingle-molecule conductance deduced from the amplitude of these tunnelcurrent fluctuations yielded values for the conductance of alkane thiolsthat were in good agreement with the break junction method. The presentinventors have made telegraph-noise measurements made by forming a fixedgap between a probe functionalized with a DNA base and a surfacefunctionalized with nucleosides. See Example 4.

Accordingly, the present invention provides a method of detecting atunnel current signal that switches between two levels, which ischaracteristic of an interaction with a single base, and wherein saidsignal is used to identify the target base in the tunnel gap. Thepresent invention also provides a method of identifying a target base ina tunnel gap by detecting a tunnel-current signal that switches betweentwo levels, wherein the signal switch is characteristic of aninteraction with a single base.

GLOSSARY

Base-Reader (BR): A class of molecule, natural or manmade that containsa predetermined pattern and spacing of Hydrogen bond donors andacceptors fixed in space on a molecular scaffold to allow the moleculeto bond and recognize molecules with complementary patterns and spacingof Hydrogen bond donors and acceptors.

Universal-Base-Reader (UBR): A class of molecule, natural or manmadethat contains sufficient predetermined pattern and spacing of Hydrogenbond donors and acceptors fixed in space on a molecular scaffold toallow the molecule through conformational changes to bond and recognizeall molecules of interest with complementary patterns and spacing ofHydrogen bond donors and acceptors.

Adenine-Base-Reader (ABR): A class of molecule, natural or manmade thatcontains a predetermined pattern and spacing of hydrogen bond donors andacceptors fixed in space on a molecular scaffold to allow theBase-Reader to bond and recognize adenine (abbreviated A). A fewexamples of an ABR class of molecule are thymine (T), uracil (U) andRiboflavin. These molecules have complementary patterns and spacing ofHydrogen bond donors and acceptors to recognize adenine (A).

Cytosine-Base-Reader (CBR): A class of molecule, natural or manmade thatcontains a predetermined pattern and spacing of Hydrogen bond donors andacceptors fixed in space on a molecular scaffold to allow theBase-Reader to bond and recognize cytosine (abbreviated C). A fewexamples of a CBR class of molecule are guanine (abbreviated G) andisoguanine. These molecules have complementary patterns and spacing ofHydrogen bond donors and acceptors to recognize cytosine (C).

Guanine-Base-Reader (GBR): A class of molecule, natural or manmade thatcontains a predetermined pattern and spacing of Hydrogen bond donors andacceptors fixed in space on a molecular scaffold to allow theBase-Reader to bond and recognize guanine (abbreviated G). A fewexamples of a GBR class of molecule are cytosine (C) and5-Methylcytosine. These molecules have complementary patterns andspacing of Hydrogen bond donors and acceptors to recognize guanine (G).

Thymine-Base-Reader (TBR): A class of molecule, natural or manmade thatcontains a predetermined pattern and spacing of Hydrogen bond donors andacceptors fixed in space on a molecular scaffold to allow theBase-Reader to bond and recognize thymine (abbreviated T) and or uracil(U). A few examples of a TBR class of molecule are adenine (abbreviatedA) and Coenzyme A. These molecules have complementary patterns andspacing of Hydrogen bond donors and acceptors to recognize thymine (T)and uracil (U).

Base Pair Reader: A class of molecule, natural or manmade that containsa predetermined pattern and spacing of Hydrogen bond donors andacceptors fixed in space on a molecular scaffold to allow the Base PairReader to bond and recognize a G-C or C-G pair or a A-T or T-A pair.

For additional glossary of terms, see pages 73-77 of U.S. provisional61/103,019.

EXAMPLES Example 1 DNA Translocation Through Single-Walled CarbonNanotubes

To test the possibility that tunnel signals could be generated bysmaller CNTs, we built a nanopore-like device (FIG. 34) was built usinglithography. Commercially-available MWCNTS (4 to 5 nm diameter) werespread on a silicon wafer, located relative to fiducial marks on thechip using low-voltage SEM, then covered in 700 nm of PMMA. E-beamlithography was used to create a series of wells in the PMMA lying onthe path of a CNT (FIG. 34B). The exposed regions of the CNT in thewells were removed with an oxygen plasma, leaving intact CNT segmentsthat connect adjacent wells. The device was completed with a molded PDMSmicrofluidic cover that allowed injection of fluids into and out of thereservoir wells (FIG. 34C). SWCNT (<1 to 2 nm diameter) devices weremade by CVD growth from Co nanoparticles followed by the same set oflithographic steps.

5 nm MWCNT tubes showed fast (μs—limited by the electronics) currentblockade pulses when 60 nt ssDNA oligomers were introduced into the 2MKCl electrolyte (FIGS. 35 A and B). Signals from the SWCNT tube deviceswere quite different. Introduction of the oligomers resulted in muchlonger pulses (30 ms) of increased current (FIGS. 35 C and D).Translocation events were much rarer (a spike per minute in the exampleshown, increasing to 10 spikes/sec at 0.5V). In order to confirm thatthese unusual pulses were indeed associated with DNA translocations, wecarried out quantitative PCR (qPCR) on aliquots collected from theoutput reservoir. Control samples (“(−) control”) were collected priorto initiating translocation. A typical qPCR output is shown in FIG. 36A.The (−) control shows no DNA even after 45 cycles. Samples collectedafter translocation gave the green curves (0.5 h translocation) and thered curve (2 h translocation). A (+) control (blue curves) was used forcalibration. Data were quantified by correcting for dilution and filterlosses (the main source of uncertainty) and a plot of the number ofmolecules translocated vs. the number of spikes for four differentdevices is shown in FIG. 36B. The amount of DNA translocated is indeedapproximately proportional to the number of translocation spikes,however, the slope of this plot implies that about 100 moleculestranslocate for each pulse.

We carried out control experiments using devices with unopened SWCNTsand devices exposed to oxygen plasma but containing no tube. Only indevices containing cut SWCNTs did we see spikes (with the correspondingDNA translocation). In addition, we found that (a) Translocationrequires at least 0.2V across the tube, the frequency and intensity ofspikes increasing as the bias is increased. (b) Translocation requiresat least 50 mM KCl, and the frequency of spikes increases with saltconcentration above this threshold. (c) Ion mobilities appear to beenhanced by about 2 orders of magnitude in the SWCNTs, consistent withthe outcome of membrane experiments. (d) Pulse widths and intensitiesvary considerably from tube to tube and even within a run, but aregenerally on the order of 30 ms with the intensities being about 10% ofthe background conductance. (e) Pulse widths do not change significantlywith bias.

We also measured the electronic properties of the SWCNT directly in adevice with two palladium electrodes crossing the tube underneath thePMMA barrier (FIG. 37A). This tube was semiconducting, and we used theunderlying silicon as a back-gate to turn the tube on (red trace FIG.37B). However, once the inside of the tube was wetted (blue and greentraces) it remained “on,” only returning to its semiconducting statewhen water was removed in high vacuum (black trace). Thus, it appearsthat the tubes are all conductive (being either metallic or “turned on”semiconductors). This unexpected result has some precedent in the betterknown adsorbate-induced surface conductivity of diamond. This wouldimply that there is little electric field inside the tube to move DNAonce it has entered. Thus the DNA may stick in the tube until rammed infurther by another molecule entering. If the tube had to fill and/orempty cooperatively, this could account for the 100:1 ratio of moleculesto translocation spikes, since the ratio of the length of the tube (2μm) to the length of the DNA (20 nm) is also 100:1. Clearly thishypothesis can be tested using different oligomer lengths and differentCNT lengths. This mechanism has obvious implications for the control oftranslocation.

Example 2 Synthesis of Trans-Base-Pair Readers with Amide and ThiolLinkers

Propargyl amine and propargyl thiol are used as the molecular linker forattachment of the trans-base-pair readers to CNT and metal electrodesrespectively. Our S™ data has shown that the propargyl group is asuitable molecular linker in terms of conductivity and flexibility.These linkers are synthesized and incorporated into the trans-base-pairreaders in their amine and thiol protected forms:N-propargyltrifluoroacetamide and propargyl S-thiobenzoate. We havedeveloped an in situ method to remove these protecting groups before usewithout need of further purification. The Sonogashira Reaction will beemployed for incorporation of the molecular linkers into thetrans-base-pair readers as discussed in the following sections.

Synthesis of molecular linker functionalized[1,8]Naphthyridine-2,7-diamines

[1,8]Naphthyridine-2,7-diamine has been synthesized using a procedurereported in literature. We have developed a facile method to incorporatepropargyl S-thiobenzoate into the β position of the naphthyridine asshown in Scheme 1 in FIG. 40. The naphthyridine was first iodinated withiodine and then reacted with propargyl S-thiobenzoate under theSonogashira conditions. Apparently, the yield of the first step isrelatively low. We will optimize the reaction conditions for scaling up.We will extend the method to synthesis of propargylaminefunctionalized[1,8]Naphthyridine-2,7-diamine. Meanwhile, we will developa method to introduce the molecular linker at the γ position of thenaphthyridine. We will design improved trans-base-pair readers assuggested by modeling.

Synthesis of molecular linker functionalized 9H-pyrrolo[3,2-h]cinnoline

The basic structure of a GC trans-base-pair reader (FIG. 38B) is fusionof cinnoline with pyrrole or indole with pyridazine, which poses morechallenges to its synthesis than the synthesis of the ATbase-pair-reader. We have worked out a number of synthetic routes forthe synthesis of the GC base pair reader. One of the possible routes isdelineated in Scheme 2 (FIG. 41). Cinnolin-8-amine is used as a startingmaterial, which can readily be synthesized according the publishedprocedure. The first step is to brominate the starting material withdibromodimethylhydantion, followed by reduction of the amine tohydrazine. The pyrrole-2-ethyl carboxylate will be constructed on thecinnoline moiety by means of the classic Fischer reaction. Afterprotecting the NH of the pyrrole ring, the ethyl ester will behydrolyzed to the acid. The carboxylic acid will be converted toBoc-amine by means of the Curtius rearrangement under the conditionsreported by Lebel and Leogane. The incorporation of the molecular linkerwill be finished in the same way as mentioned above. Both PMB and Bocprotecting group can be removed with trifluoroacetic acid simultaneouslyto complete the synthesis.

NMR Studies on Interactions of the DNA Trans-Base-Pair Readers with theDNA Base-Pairs.

We will conduct NMR recognition studies of the base pairs by these newsynthetic trans-base-pair readers in an aprotic solvent, such aschloroform. NMR titration and temperature dependent measurement allowsus to determine association constants, association enthalpies andentropies, and 2D NOE may allow us to establish the hydrogen bondingmodes. Four natural nucleosides will be used as target molecules. Allthe nucleosides, base readers and trans-base-pair readers will bemodified to improve their solubility in the NMR solvent withoutimpairing their hydrogen bonding capacities. First, we will studyinteractions of base readers with four natural nucleosides, determiningthe base reading specificity and affinity. Next, we will study the basepair recognition by trans-base-pair readers. We expect to achieve twogoals from the NMR studies: (1) establishing thermodynamics of therecognition of base and trans-base-pair readers; (2) confirming theirrecognition patterns. Combining chemical information with theirelectronic properties will allow us to design the next generation oftrans-base-pair readers using computer modeling.

Example 3 Attachment Schemes

Design and Modeling of Trans-Base-Pair Reader Reagent AttachmentSchemes.

Oxygen-plasma etching of CNTs leaves the ends predominantlyfunctionalized with carboxylic acid residues, so amide linkages canreadily be made. However, our device requires different reagents on eachelectrode. There are currently three options, each of which will beexplored as required to produce the required degree of functionalizationof each electrode:

Random Functionalization.

There are many sites available on the ends of 2 nm diameter tubes (FIG.39). If the probability of each reagent binding a particular site isequal, then on the 18,0 tube shown, the probability that each site on agiven tube binds just one of the two reagents is (0.5) which isvanishingly small. In practice, not every site will be reacted and thereis the possibility of fruitless assemblies with the target base acrossthe end of one tube. These may not matter if fluctuations drive anassembly across the electrodes frequently enough.

Electrochemical Functionalization.

The use of electrochemistry to generate a reactive species only in theimmediate vicinity of one electrode is a well-developed technology. Thechallenge here lies with confining electrochemically-generated reactivespecies to just one electrode in a gap of nanometer dimensions.Selective electrochemical functionalization of SWCNTs by reduction ofaryl diazonium salts has been widely used since its introduction and itis claimed to be localized to within 1 nm. The diazonium radicalattaches to any sp² hybridized carbon so the process is not veryselective, though it does appear to functionalize the ends of tubespreferentially. This will be one approach we will try. A more attractivepossibility is the oxidation of hydroquinone-modified tubes, to producebenzoquinone as a good leaving group. This scheme is shown in FIG. 46.The hydroquinone is first coupled to the carboxylate residues on the CNTwith EDC or DCC chemistry. It is selectively oxidized on one electrodeto benzoquinone in the presence of the amine-terminated trans-base-pairreader, which is attached to the CNT by an amide linkage in one step.The electrode to be functionalized will be poised at the oxidationpotential with respect to a reference electrode. The process will berepeated on the second electrode with an amine-functionalized base. Weexpect that oxidation potentials will be modified a little by the closeproximity of the second electrode but the effect can be minimized byoperating at high electrolyte concentration.

Analysis of the functionalization is also challenging on this lengthscale. Goldsmith et al. describe some remarkable electrochemicalmeasurements on SWCNTs that lead us to believe that useful electricalsignals may be obtained even on these small length scales. They alsodescribe an ingenious approach for detecting modified sites based onimaging bound functionalized gold nanoparticles in an SEM, a techniquewe will apply here.

Metal-CNT Gap.

Below we outline an alternative strategy for fabrication of a metal-CNTtunnel gap. This has the advantage of eliminating all nanofabricationsteps and making small (nm) gaps routinely between a CNT and a metalelectrode. Orthogonal attachment chemistries are then readily available(e.g., amide linkages to the CNT and thiol linkages to the metal).

A CNT to Metal Electrode Gap.

We have a method to create small gaps using an overhanging oxidizedlayer on aluminum, illustrated in FIG. 47 and described for the case ofmetal-metal junctions elsewhere. The result is a device in which a Ptelectrode faces a CNT over a gap of a few nm. These devices are fairlystraightforward to fabricate, and we will first characterizetranslocation in such devices using the methods described above. Ifreliable translocation is obtained, we will then proceed tofunctionalize the devices using amide linkages to the CNT and thiollinkages to the Pt. We will characterize the devices and compare theirperformance to devices based on CNT gaps.

Characterize Base Reading and Incorporation of Translocation Control.

We will use nucleotides to characterize signals arising from the captureof matched and mismatched targets, much as was described for gold-goldjunctions, comparing the signals to the results of the simulationsdescribed above. We have allowed for one complete cycle of refinement(theoretical modeling and design, synthesis and characterization) drivenby the insights gained from these first rounds of tests. This will alsorepresent our first opportunity to check the interplay betweentranslocation bias and reading bias and we will study these effects withsmall oligomers. We have emphasized changes in conductance as a readoutmechanism, but we expect changes in the lifetime of the bonded complexto be an important signal too. We further expect these lifetimes to beexponentially-sensitive to the force pulling on the DNA. We willcharacterize the interactions between translocation and readout tocharacterize the way in which the readout is affected by theelectrophoretic force on the translocated DNA and compare our results tocomputer models

Example 4 Determining Tunnel Conductance of a Single Hydrogen BondedPair

Here, we report on “telegraph-noise” measurements made by forming afixed gap between a probe functionalized with a DNA base and a surfacefunctionalized with nucleosides.

We synthesized the nucleosides 5′-S-acetyl-5′-thiodeoxycytidine and5′-S-acetyl-5′-thiothymidine following published protocols. Thesecompounds were employed to prepare the nucleoside monolayers onfreshly-prepared Au(111) substrates using the method developed in ourlab. Gold STM probes were prepared and functionalized with8-mercaptoadenine (A), 2-amino-8-mercaptoadenine (2-AA) and8-mercaptoguanine (G) as previously described. The Watson-Crick basepairings are shown below for (i) adenine-thymidine, (ii)2-aminoadenine-thymidine and (iii) guanine-deoxycytidine.

We also prepared STM probes and gold substrates functionalized with amonolayer of thiophenol as controls. Further controls employed bare goldprobes or substrates. Measurements were carried out on a PicoSTM(Agilent, Chandler) with the sample and probe submerged in1,2,4-trichlorobenzene. The system was first left to stabilize for threeto four hours, and then the probe was advanced towards the surface toachieve a chosen set-point current (I_(SP)) at a tip to substrate bias,V_(b). Immediately after the set-point was achieved, the servo-controlwas broken using custom LabView software, and the tunnel currentrecorded as a function of time using a digital oscilloscope. The tunneljunctions remained stable without servo control for up to ten seconds.

In the hydrogen bonded systems, bursts of “telegraph noise” like thatshown in FIG. 48 d were recorded in about half the data collection runs.Usually (>95% of the data collected) the noise reflected stochasticswitching between just two distinct levels, indicative of a singlebase-nucleoside pair fluctuating in the junction. Some bond-breakingpossibilities are illustrated in FIG. 48. The molecule-metal contactsmay “break” (FIG. 48 b) as in the case of similar recordings from simplebis-thiolated molecules. The motion in the figure is exaggerated foreffect, for the frequent re-connection suggests only a small motion ofthe contact (which is probably not at the Au—S bond, but rather at theAu—Au bonds that surround the Au atom attached to the S atoms—motion inmany such thiol-tethered systems is also affected strongly byinteractions with the surrounding molecular matrix.) Simultaneousbreaking of all the hydrogen bonds (FIG. 48 c) is yet anotherpossibility for the systems studied here, and it appears that bothprocesses contribute to the observed telegraph noise, as evidenced by abimodal distribution of switching times (see below).

The result of a typical control experiment (thiophenol probe, thymidinemonolayer) is shown in FIG. 49 a with a current trace for 0.5 s worth ofdata on the left, and a histogram of the current distribution on theright. The current noise is distributed evenly around the mean. Incontrast, the H-bonded systems (FIGS. 49 b, c and d) all show distincttelegraph noise with a corresponding bimodal current distribution. Wecharacterized the frequency with which telegraph signals occurred asfollows: once the instrument was stabilized and a signal first recorded,we measured the fraction of each 5 s run occupied by telegraph noise.These fractions are shown as percentages in Table 1. For controlexperiments where telegraph noise was not seen at all (“0” in Table 1)we followed the same stabilization procedure and then recorded currentdata for the same overall experimental duration. Interestingly,thiophenol on the probe or substrate did show rare (few percent)instances of telegraph noise in contact with a base or nucleoside butonly at the very lowest bias used. Presumably, interactions between thearomatic benzene ring and the heterocycle can occasionally result incomplexation. These rare exceptions aside, the experiments summarized inTable 1 show that the entire metal-thiol-molecule-H-bonds-molecule-metalsystem needs to be assembled for telegraph noise to be observed.

TABLE 1 Observed frequency of switching (fraction of time occupied bytelegraph noise in a 5 s run) for various preparations of the probe(left column) and the surface (top row). Thio- Thio- thymidinedeoxycytidine Thiophenol Bare 8-Mercapto- 47 ± 8% — 1 ± 0.3%* 0 adenine8-Mercapto-2- 51 ± 7% — — 0 aminoadenine 8-Mercapto- — 52.5 ± 13% — 0guanine Thiophenol  2.5 ± 1%* — 0 0 Bare 0 0 0 0 Adenine 0 — — —2-Aminoadenine 0 — — — Errors are ±1sd. “—” represents untriedcombinations. *indicates that the observed switching only occurred atlow bias (50 mV). The bottom two rows are for bases physisorbed onto theprobes (no thiol attachments).

Haiss et al. have found that the measured conductance of a rigidmolecule increased approximately exponentially as the tunnel gap wasmade smaller, an effect they attributed to movement of the molecularlevels relative to the Fermi level as the molecule is rotated towardsthe surface. In the present, more complex system, the measured switchingconductances display a variety of dependences on the baselineconductance (G(bl) in FIG. 49), shown for the three types of molecularpair in FIG. 50. We have chosen to plot the switching conductance vs.G(bl) rather than against the gap value itself because of the rathercomplex relationship between gap and conductance in these systems.Plotted this way, an exponential increase in conductance with gapappears as a linear increase of conductance with G(bl). In all cases,the switching conductance increases rapidly with G(bl) at first (grayshaded boxes, FIG. 50 a-c). Above a critical conductance (similar in allthree cases, and marked by an arrow) the switching conductance takes ona broad range of values. The upper limit clearly increases linearly withG(bl), consistent with the tilting mechanism described by Haiss et al.(though other types of distortion are surely also possible in this morecomplex system). However, about half of the total data points (greenshaded boxes) lie on a plateau of constant conductance. Oneinterpretation of these data is shown in FIG. 50 d. In the regionlabeled “1”, the gap is too large to be spanned by molecular pairs attheir equilibrium spacing, so molecular-pairs that do span the gap arestrained, with a conductance that falls exponentially with strain (i.e.,falls linearly as G(bl) falls). Once the gap becomes equal to the lengthof an unstained molecular pair, further decreases in the gap result inan increase in molecular switching conductance, possibly owing to thetilting mechanism described by Haiss et al. This is the region labeled“2”. However, the smaller gaps should also accommodate a number ofmolecular-pairs of equilibrium length at some critical radius of theprobe (r_(c) in FIG. 50 d), resulting in a plateau of constantconductance (“3”, shown as the green shaded region). The onset of thisplateau will occur when the gap can first just accommodate an unstrainedmolecule, presumably at the point marked by the arrows on FIG. 50.Molecules attached at intermediate points in smaller junctions(0<r<r_(c)) will generate conductances that lie in between these twolimits, generating data points above the plateau, but smaller than themaximum values measured. Haiss et al. discuss these possibilities, butconclude that their experiments capture molecules only at the minimumgap, because molecules are pulled into this region by the tip-sampleelectric field. Our experimental arrangement is different, because onehalf of the molecular pair is attached to the probe before the junctionis made. Thus, we believe that all geometries can be captured in gapssmaller than the critical value. It is also possible that otherarrangements such as bent nucleoside-base-pairs could occur. Bothgeometry (the number of available sites increasing with available tipradius) and energetics (equilibrium molecular pairs have the lowestenergy) favor equilibrium pairs, and we find that about half the totaldata points lie in region 3.

Support for two different types of bonding (stretched vs. equilibriumand equilibrium but tilted) can be found in an analysis of the switchingtimes. The peaks in all the bimodal current histograms are wellseparated, so the duration of the on-state is easily extracted byrecording the intervals for which the current remains above 50% of theswitching range in a given run. We plotted distributions of theseswitching times using logarithmic histograms and data for theG-deoxycytidine pair are shown in FIG. 51 (similar plots for the otherbase-nucleotide combinations are shown in FIG. 54). The distribution ofswitching times is clearly also bimodal. We fitted Gaussians to the twopeaks (“F”=fast, ˜8 ms, “S”=slow, ˜50 ms) and the fitting parameters arelisted in Table 2 for all three base-nucleoside combinations. The fastpeak is somewhat faster and noticeably more prominent in junctions heldtogether by two hydrogen bonds (A-thymidine) compared to junctions heldtogether by 3 (G-deoxycytidine, 2AA-Thymidine). Further, the timescalesare very similar to the characteristic times for opening of DNA bases ina double helix. Thus, the fast process appears to be a consequence ofH-bond opening of the complex (FIG. 48 c). The slow peak does not appearto change with H-bonding. In view of the similarity with the timescalefor opening of thiol-gold contacts we ascribe this feature tofluctuations in the molecule-metal contact (FIG. 48 b). The distributionof switching times is broken down as a function of baseline conductancein the 2D plot on the right of FIG. 51 (and see FIG. 54).

TABLE 2 Measured conductances obtained from the conductance plateauregion (“3” in FIG. 3) compared to calculated values (G_(pred)).Base-nucleoside pair G_(meas.) (nS) G_(pred.) (nS) τ_(F) (ms) τ_(S) (ms)h_(F)/h_(S) A-thymidine 0.76 ± 0.01 (±0.03) 1.62   7 ± 3.6 46 ± 11 1.3 ±0.1   2AA-thymidine 0.80 ± 0.03 (±0.14) 1.43 8.3 ± 3.9 45 ± 11 1 ± 0.1G-deoxycytidine 0.66 ± 0.007 (±0.07) 0.96 8.4 ± 3.4 47 ± 11 1 ± 0.1Values are best-fits to the I-V curves in FIG. 5 with fitting errorsshown. Error values in parenthesis are derived from the SDs of thesingle molecule measurements (spread of data in the green boxes in FIG.50). Also shown are mean lifetimes (τ_(F), τ_(S)) with associatedhalfwidths, and ratios of the peak heights, h_(F)/h_(S), for Gaussianfits to the switching time distributions.

The fast process diminishes rapidly with increasing baselineconductance, while the slow process becomes more important withincreasing baseline conductance. The turnover, which starts in theneighborhood of G(bl)=1 nS, begins in about the same place that themolecular switching conductance moves from region 1 to regions 2 and 3(FIG. 50). This is consistent with the idea that the hydrogen bonding ismore stable (less strained) when the molecular pairs span junctionssmaller than the equilibrium length of the pairs. The “disappearance” ofH-bond openings in very small gaps is unexpected, and possibly reflectsthe consequences of confining the molecular pair to a very small volume.

If this interpretation is correct, then the conductance of the relaxedmolecular pairs will be given by the data in the plateau regions in thegreen boxes (region 3) on FIG. 50. Using these data, we plot the meancurrents (error bars are ±1sd) as a function of V_(b) in FIG. 52. Thecurrent-voltage relationships for all three Watson-Crick bonded pairsare linear, and the slopes yield the conductances listed in Table 2.Density-functional methods have been used to calculate the conductancesof these base-nucleoside pairs and the results of these calculations arereproduced in the third column (G_(pred)) of Table 2. It is strikingthat the conductances obtained from the plateau regions agree with thepredicted conductances within a factor of two, remarkably good agreementin the field of single molecule conductance measurements. Even morestrikingly, the surprising prediction that G-deoxycytidine (3H-bonds) isless conductive than that A-thymidine (2H-bonds) is supported by thisanalysis. (This unusual result is, in the theory, a consequence of thedifferent relative position of the Fermi-level in the two cases.).

We have determined the absolute tunnel conductance for A-Thymidine,2AA-Thymidine and G-deoxythymidine spanning gold electrodes. Further,the analysis of switching conductance as a function of baselineconductance facilitates design of an optimal fixed tunnel gap forreadouts of DNA basepairs. Clearly, the gap must be smaller than thecritical value at the transition from region 1 to regions 2 and 3. Theclose correspondence between the measurements and theory for the presentcase of base-nucleoside interactions adds credibility to the DFTcalculations. These same calculations predict very small (fS)conductance across an entire DNA molecule. Thus, electronic sequencingof DNA probably requires readout schemes that involve shorter tunnelingpaths, and these schemes are currently under investigation in ourlaboratory.

Electronic Characterization of Trans-base-pair Readers.

We will use the telegraph-noise measurement perfected above tocharacterize the trans-base-pair readers with, and without targetnucleotides in a fixed gold electrode tunnel gap (FIG. 42). Measurementswill be carried out in buffered aqueous electrolyte, and we will firstobtain baseline data for the reader-pair alone (FIG. 42A, B). We expectto see significant differences when nucleotides are introduced into thesolution and bind to the recognition molecules (FIG. 42C, D). Thiselectronic detection of a single nucleotide is a significant milestone,and the resulting signals will yield a distribution of single moleculeconductances and lifetimes for the various complexes. These can becompared to the theoretical predictions (for gold-gold junctions) andused to help refine the models discussed above. We will measure signalsfrom each of the four nucleotides, and characterize all of the 12possible mismatches, once again comparing the data to the predictions,refining calculations as needed. We will determine the effects of highsalt and strong buffers, and test the consequences of chemical changes,such as moving the linker attachment site, once again, comparing ourmeasurements to the predictions of theory. These iterations will allowus to refine the theory for the gold-gold junctions, giving us a betterstarting point for modeling behavior in a CNT gap. The two systemsdiffer substantially (probably requiring different choices ofexchange-correlation functional) but the lessons learned from thegold-gold junctions (which we presently have working) will surely bevaluable to us in designing the less-familiar CNT gaps.

Example 5 Characterization of DNA Translocation through “Tight” SWCNTs

Testing translocation of DNA through tight SWCNTs experimentally willrequire translocation of long molecules. As a first step, we willfurther explore translocation of small oligomers. Our present modelpredicts that the number of molecules translocated per signal pulse,N_(T) is given by

$N_{T} = \frac{L_{CNT}}{L_{DNA}}$and this can be tested by varying the length of the SWCNT, L_(CNT)and/or the length of the oligomer, L_(DNA), and measuring the amount ofDNA translocated with qPCR. The shortest DNA length we can use islimited by the required PCR primers, while the longest is limited by theprecipitous drop in translocation rate as oligomer length is increasedin a non-denaturing environment. In practice this allows for a factor 3in DNA length (60 nt to 180 nt) while tube lengths can be changed over alarger range (2μ to at least 20μ). We do not understand the origin ofthe positive current spikes in tight tubes (FIG. 35D) but expect thatinsights will come from the use of salts other than KCl (e.g., LiCl,NaCl, CsCl). Characterization of the spikes is a key requirement fordeveloping feedback control of the translocation process. Progress willbe enhanced by improving the yield of devices. The block-copolymercatalyst we currently use yields a most probable tube diameter of about1 nm, with only a fraction the required 2 nm. Better control of CNTposition and size would also shorten the manufacturing process. We willadopt the method reported by Dai et al. to grow SWCNTs at specificlocations with specific sizes on an Si wafer. This approach uses e-beamlithography to pattern nanometer sized holes on the photoresist coatedsubstrate. A thin film of Co and Fe is deposited by e-beam evaporation.After lift-off, the substrate is annealed at high temperature. Duringthe annealing, the catalyst atoms diffuse to form small nanoparticlesthat can be used as SWCNT catalysts. By controlling the diameter of thee-beam patterned hole and the thickness of the catalyst film, thediameter of the nanoparticle is controlled.

Building a Model of Transport in “tight” SWCNTs.

We will perform molecular dynamics simulations to compute the optimalparameters for controlling the motion of a DNA molecule through theinterior of the SWCNT as well in the high-field region near the entranceto the tube. The parameters of interest for given dimensions of theSWCNT and the DNA are voltages applied across the reading gaps, theelectric field for the DNA electrophoresis through the SWCNT, and theconcentration of electrolytes. These simulations will be based on priorexperience at Oak Ridge in simulating DNA in aqueous solution. We willuse the force fields based on AMBER, UFF and Tersoff-Brenner potentials(REBO, AIREBO). The polarization effect of the conducting CNT will beincluded using the ECD method. The large scale molecular dynamicspackages NAMD and NWCHEM will be used to solve the equations of motion.These codes scale well with large number of processors and areappropriate for large scale calculations. We will perform a series ofcomputational simulations as follows: (1) Perform MD simulations of DNAmolecules in aqueous solution of the SWCNT to determine the optimalconditions for the DNA translocation through the pore at an appropriatespeed. We anticipate these will be molecular dynamics simulations in atime scale of tens or hundreds of nanoseconds. (2) Project the resultsto longer time scales. The ideal signature detection timescale inexperiments may be well beyond the timescale that standard MD can reachwith the current state-of-the-art computational facilities. A series ofconstrained simulations will be performed at various steps as themolecule moves through the detection gate to measure the external forcesrequired to maintain its translocation and orientation. This will yielda force profile for determining the lower bound for the feasibility ofan actual device. The calculations for longer times (up to ms andlonger) rely on the solution of the master transport equation (ofLiouville type), a 6-dimensional 2nd order partial differential equationin configuration space of representative coordinates and momenta, whichwe will solve by well known Monte Carlo techniques. Although thecalculations in this approach are much less intensive, these criticallydepend on the input (sources and sinks) provided by the reaction rates,diffusion coefficients and dynamical variables, obtained by MD and fromother methods, including experiments.

Build a Device with a Nano Gap and Test Translocation.

DNA is clearly capable of exiting a “tight” SWCNT, but what happens ifanother CNT is close by, as in the proposed reading gap? As DNAmolecules approach the nanometer-size gap in the SWCNT, they have twopossible destinies. They can either (a) move directly into the otherhalf of the SWCNT, or (b) leave the nanogap and diffuse into thesolution. We can read the DNA sequence with the “recognition molecules”in both cases, but translocation across the gap would enable sequentialreads of all four bases on each molecule (FIG. 43). We will build thethree-well device outlined in FIG. 44. In this device, a nanogap is cutin the SWCNT using e-beam lithography. DNA will be injected into theleft reservoir and allowed to translocate through the SWCNTs. Solutionsin the other two reservoirs will be collected and analyzed with qPCR toquantify the number of DNA copies translocated into each reservoir as afunction of the electrophoretic driving field, V_(E). Further controlcan be added by using a third reference electrode to control thepotential of the middle reservoir. Finally, a device with electricalcontacts (FIG. 45) will be used to probe the effects of the reading biasapplied across the gap (a higher bias should enhance trans-gaptranslocation). These data will be used to refine our computer model oftranslocation, so that we can find optimal biases for both translocationand readout.

Active Translocation Control.

The positive current spikes characteristic of translocation through“tight” CNTs span 5 to 30 ms in measurements made to date. These signalsappear to be a consequence of the cooperative emptying of the tubes, andso may be determined more by the tube itself than the length of the DNA.Whatever their origin, the signals are much longer than the 100 μsresponse of our measurement system (FIG. 35D). We will build afeedback-controlled bias source, designed to switch the bias across thetube to zero at a chosen interval after a translocation signal isdetected. If the DNA remains stuck in the tube, we expect to seeevidence of this in a current signal that differs from the backgroundonce the bias is restored. If we can do this, we should be able todetect signals as the same molecule is moved back and forth through thenanopore, much as has been done with a conventional nanopore. We willcarry out a systematic study of signals as a function of DNA length,bias, CNT length and diameter and salt concentration. It may provepossible to detect a signal representative of the purine/pyrimidinecontent of various sequences if specific chemical interactions with thewall of the tube differ enough between different natural sequences. Whencombined with the theoretical simulations described above we hope tounderstand how to control the dwell time on each base, how to minimize“stutter” (the DNA goes backwards), how uniform (or otherwise)translocation speed is, and how to do multiple reads on the samemolecule.

Example 6

A number of types of “universal reader” molecules are discussed above.These have in common the ability to form hydrogen bonds with all fourbases, and to do so in a way that a tunnel gap will be spanned when abase in bonded by the same reagent on both surfaces. The use of the samereagent on both electrodes is an important simplification that can beachieved with the use of certain universal readers. A generic feature ofthese molecules is that they posses both hydrogen bond donors andacceptors, spaced in a way that allows them to form hydrogen bonds withmultiple sites on the DNA bases. Thiobenzoic acid (see FIG. 58B) is asimple molecule which can act as a hydrogen bond donor and acceptor inaprotic solvents (i.e., organic solvent without free proteins). This isbecause the carboxylate residue remains protonated as shown in FIG. 58B.The OH group acts rather like a hydrogen bond donor and the doublybonded oxygen is similar to a hydrogen bond acceptor so that thesemolecules are capable of quite strong pairing with each other.Monolayers of thiobenzoic acid were made on Au(111) surfaces, and theirdensity, height and orientation was verified with STM imaging, FTIR andellipsometry. Contact angle measurements showed the films to be highlyhydrophilic in a humid atmosphere, confirming that the surface wasterminated by the carboxylate residue. To make nucleosides soluble in anorganic solvent (trichlorobenzene, TCB), the OH groups in thedeoxyribose ring were functionalized with t-butyldimethylsilyl (TBDMS).This was performed for all four nucleosides and also 5-methyl cytidine.FIG. 58A shows the modified adenosine as an example.

A gold probe was functionalized with thiobenzoic acid and a tunnel gapover a functionalized gold surface submerged in freshly distilledtrichlorobenzene (TCB) in an STM was established. At high tunnelcurrents, signals were noisy, indicative of interactions between thebenzoic acid residues on the probe and substrate. However, when the gapconductance was reduced below 100 pS, the signal became quiet (FIG. 59Ashows data for a baseline conductance, G_(BL), of 12 pS). Whennucleosides were flowed into the liquid cell, sharp spikes appeared asshown in FIG. 59B. The distribution of spike heights depended upon thesize of the tunnel gap, as might be expected, given the analysis oftelegraph noise shown in FIG. 50. Data for cytidine and thymidine arepresented in FIG. 60. At very large gaps (G_(BL)=4 pS) very few eventsare recorded. Many more are recorded at G_(BL)=12 pS. The numberincreases again as the gap 1s decreased to give G_(BL)=20 pS, but thewidth of the distribution increases substantially, again, in line withthe trends displayed in FIG. 50.

Spike distributions for all four nucleosides and 5 methyl-cytidine areshown in FIG. 61A. It is clear that the nucleosides can be read withsome degree of discrimination. A single tunneling read holds promise forrecognizing all four nucleotides as well as 5 methyl-cytidine.

FIG. 61B superimposes data taken from adenine, superimposed on top ofthe distribution obtained from adenosine. The peaks are in the sameposition, showing that the base dominates the tunneling. However, thedata for adenosine are substantially broader, suggesting thatfluctuations involving the sugar ring contribute to the spread of thedata.

Finally, the degree of discrimination obtained here is much better thanthat theoretically predicted for tunneling through bases withoutfunctionalized electrodes. This point is demonstrated experimentally,with measurements of the spike distribution made with bare goldelectrodes (and G_(BL) increased to 40 pS to get a signal). The resultsare shown for cytidine and thymidine in FIG. 62.

The invention claimed is:
 1. A molecular recognition chip configured toidentify at least one portion of a target molecule, the chip comprising:a) a substrate; b) a first nanotube passing through at least a portionof the substrate, the first nanotube provided with a nanopore andconfigured and dimensioned to permit the translocation of a targetmolecule therethrough; c) a second nanotube supported by the substrateand disposed in a fixed relationship with respect to said first nanotubeso as to form a first gap with the first nanotube; and d) an electricalcircuit configured to detect an electrical current between the first andsecond nanotubes, upon passage of the target molecule past said gap;wherein the first nanotube comprises a first recognition elementconnected to an end thereof, the second nanotube comprises a secondrecognition element connected to a first end thereof, and wherein eachof the first and second recognition elements further comprise auniversal base reader.
 2. The chip of claim 1 wherein the universal basereader is capable of recognizing different DNA bases and is also capableof forming a set of hydrogen bonds with a complex of a DNA base and theuniversal base reader.
 3. The chip of claim 2 wherein the base readercomprises 4-(mercaptomethyl)-1H-imidazole-2-carboxamide.
 4. The chip ofclaim 2 wherein the base reader comprises5-(2-aminoethyl)-1H-imidazole-2-carboxamide.
 5. The chip of claim 2wherein the base reader comprises thiobenzoic acid.
 6. The device ofclaim 5 wherein the universal base readers comprise4-(mercaptomethyl)-1H-imidazole-2-carboxamide.
 7. The device of claim 5wherein the universal base readers comprise5-(2-aminoethyl)-1H-imidazole-2-carboxamide.
 8. The device of claim 7wherein the molecular linker is functionalized with an anchoring groupselected from the group consisting of amine, thiol, carboxylic acid,halogen, selenium, and hydroxyl.
 9. The device of claim 5 wherein theuniversal base reader comprises thiobenzoic acid.
 10. The device ofclaim 5 wherein the universal base readers comprise a conjugate of aplanar n system with a functional group, amine, amide, nitro, carboxylicacid, and halogen, connected to a molecular linker or a conjugate of thefunctional group with a molecular linker.
 11. The device of claim 1,wherein the universal base readers are selected, independently, from thegroup consisting of:

wherein R is CH₂CH₂R′ or CH₂R′, and wherein R′ is SH, NH₂, or SeH.
 12. Amethod of sequencing DNA as the DNA molecule translocates through aconstriction, the method comprising: providing an apparatus comprising adevice in accordance with claim 1, a first chamber and a second chamber,the device being located in the apparatus such that the first chamber islocated on a first side of the device and the second chamber is locatedon a second side of the device; introducing a DNA molecule into thesecond chamber; electrophoresing the DNA molecule so that ittranslocates through the constriction; and detecting an electricalcurrent passing through the first electrode, a first universal basereader, a nucleotide base of the DNA molecule, a second universal basereader, and the second electrode.
 13. The method of claim 12 wherein thefirst and second universal base reader are the same.
 14. The method ofclaim 12 wherein the universal base reader comprises thiobenzoic acid.